# documentação Django 2.0.8

# Código fonte para django.db.backends.base.schema

    import hashlib
    import logging
    from datetime import datetime

    from django.db.backends.ddl_references import (
        Columns, ForeignKeyName, IndexName, Statement, Table,
    )
    from django.db.backends.utils import split_identifier
    from django.db.models import Index
    from django.db.transaction import TransactionManagementError, atomic
    from django.utils import timezone
    from django.utils.encoding import force_bytes

    logger = logging.getLogger('django.db.backends.schema')

    def _is_relevant_relation(relation, altered_field):
        """
        When altering the given field, must constraints on its model from the given
        relation be temporarily dropped?
        """
        field = relation.field
        if field.many_to_many:
            # M2M reverse field
            return False
        if altered_field.primary_key and field.to_fields == [None]:
            # Foreign key constraint on the primary key, which is being altered.
            return True
        # Is the constraint targeting the field being altered?
        return altered_field.name in field.to_fields

    def _related_non_m2m_objects(old_field, new_field):
        # Filter out m2m objects from reverse relations.
        # Return (old_relation, new_relation) tuples.
        return zip(
            (obj for obj in old_field.model._meta.related_objects if _is_relevant_relation(obj, old_field)),
            (obj for obj in new_field.model._meta.related_objects if _is_relevant_relation(obj, new_field))
        )

    class BaseDatabaseSchemaEditor:
        """
        This class and its subclasses are responsible for emitting schema-changing
        statements to the databases - model creation/removal/alteration, field
        renaming, index fiddling, and so on.
        """

        # Overrideable SQL templates
        sql_create_table = "CREATE TABLE %(table)s (%(definition)s)"
        sql_rename_table = "ALTER TABLE %(old_table)s RENAME TO %(new_table)s"
        sql_retablespace_table = "ALTER TABLE %(table)s SET TABLESPACE %(new_tablespace)s"
        sql_delete_table = "DROP TABLE %(table)s CASCADE"

        sql_create_column = "ALTER TABLE %(table)s ADD COLUMN %(column)s %(definition)s"
        sql_alter_column = "ALTER TABLE %(table)s %(changes)s"
        sql_alter_column_type = "ALTER COLUMN %(column)s TYPE %(type)s"
        sql_alter_column_null = "ALTER COLUMN %(column)s DROP NOT NULL"
        sql_alter_column_not_null = "ALTER COLUMN %(column)s SET NOT NULL"
        sql_alter_column_default = "ALTER COLUMN %(column)s SET DEFAULT %(default)s"
        sql_alter_column_no_default = "ALTER COLUMN %(column)s DROP DEFAULT"
        sql_delete_column = "ALTER TABLE %(table)s DROP COLUMN %(column)s CASCADE"
        sql_rename_column = "ALTER TABLE %(table)s RENAME COLUMN %(old_column)s TO %(new_column)s"
        sql_update_with_default = "UPDATE %(table)s SET %(column)s = %(default)s WHERE %(column)s IS NULL"

        sql_create_check = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s CHECK (%(check)s)"
        sql_delete_check = "ALTER TABLE %(table)s DROP CONSTRAINT %(name)s"

        sql_create_unique = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s UNIQUE (%(columns)s)"
        sql_delete_unique = "ALTER TABLE %(table)s DROP CONSTRAINT %(name)s"

        sql_create_fk = (
            "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s FOREIGN KEY (%(column)s) "
            "REFERENCES %(to_table)s (%(to_column)s)%(deferrable)s"
        )
        sql_create_inline_fk = None
        sql_delete_fk = "ALTER TABLE %(table)s DROP CONSTRAINT %(name)s"

        sql_create_index = "CREATE INDEX %(name)s ON %(table)s (%(columns)s)%(extra)s"
        sql_delete_index = "DROP INDEX %(name)s"

        sql_create_pk = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s PRIMARY KEY (%(columns)s)"
        sql_delete_pk = "ALTER TABLE %(table)s DROP CONSTRAINT %(name)s"

        sql_delete_procedure = 'DROP PROCEDURE %(procedure)s'

        def __init__(self, connection, collect_sql=False, atomic=True):
            self.connection = connection
            self.collect_sql = collect_sql
            if self.collect_sql:
                self.collected_sql = []
            self.atomic_migration = self.connection.features.can_rollback_ddl and atomic

        # State-managing methods

        def __enter__(self):
            self.deferred_sql = []
            if self.atomic_migration:
                self.atomic = atomic(self.connection.alias)
                self.atomic.__enter__()
            return self

        def __exit__(self, exc_type, exc_value, traceback):
            if exc_type is None:
                for sql in self.deferred_sql:
                    self.execute(sql)
            if self.atomic_migration:
                self.atomic.__exit__(exc_type, exc_value, traceback)

        # Core utility functions

        def execute(self, sql, params=()):
            """Execute the given SQL statement, with optional parameters."""
            # Don't perform the transactional DDL check if SQL is being collected
            # as it's not going to be executed anyway.
            if not self.collect_sql and self.connection.in_atomic_block and not self.connection.features.can_rollback_ddl:
                raise TransactionManagementError(
                    "Executing DDL statements while in a transaction on databases "
                    "that can't perform a rollback is prohibited."
                )
            # Account for non-string statement objects.
            sql = str(sql)
            # Log the command we're running, then run it
            logger.debug("%s; (params %r)", sql, params, extra={'params': params, 'sql': sql})
            if self.collect_sql:
                ending = "" if sql.endswith(";") else ";"
                if params is not None:
                    self.collected_sql.append((sql % tuple(map(self.quote_value, params))) + ending)
                else:
                    self.collected_sql.append(sql + ending)
            else:
                with self.connection.cursor() as cursor:
                    cursor.execute(sql, params)

        def quote_name(self, name):
            return self.connection.ops.quote_name(name)

        @classmethod
        def _digest(cls, *args):
            """
            Generate a 32-bit digest of a set of arguments that can be used to
            shorten identifying names.
            """
            h = hashlib.md5()
            for arg in args:
                h.update(force_bytes(arg))
            return h.hexdigest()[:8]

        # Field <-> database mapping functions

        def column_sql(self, model, field, include_default=False):
            """
            Take a field and return its column definition.
            The field must already have had set_attributes_from_name() called.
            """
            # Get the column's type and use that as the basis of the SQL
            db_params = field.db_parameters(connection=self.connection)
            sql = db_params['type']
            params = []
            # Check for fields that aren't actually columns (e.g. M2M)
            if sql is None:
                return None, None
            # Work out nullability
            null = field.null
            # If we were told to include a default value, do so
            include_default = include_default and not self.skip_default(field)
            if include_default:
                default_value = self.effective_default(field)
                if default_value is not None:
                    if self.connection.features.requires_literal_defaults:
                        # Some databases can't take defaults as a parameter (oracle)
                        # If this is the case, the individual schema backend should
                        # implement prepare_default
                        sql += " DEFAULT %s" % self.prepare_default(default_value)
                    else:
                        sql += " DEFAULT %s"
                        params += [default_value]
            # Oracle treats the empty string ('') as null, so coerce the null
            # option whenever '' is a possible value.
            if (field.empty_strings_allowed and not field.primary_key and
                    self.connection.features.interprets_empty_strings_as_nulls):
                null = True
            if null and not self.connection.features.implied_column_null:
                sql += " NULL"
            elif not null:
                sql += " NOT NULL"
            # Primary key/unique outputs
            if field.primary_key:
                sql += " PRIMARY KEY"
            elif field.unique:
                sql += " UNIQUE"
            # Optionally add the tablespace if it's an implicitly indexed column
            tablespace = field.db_tablespace or model._meta.db_tablespace
            if tablespace and self.connection.features.supports_tablespaces and field.unique:
                sql += " %s" % self.connection.ops.tablespace_sql(tablespace, inline=True)
            # Return the sql
            return sql, params

        def skip_default(self, field):
            """
            Some backends don't accept default values for certain columns types
            (i.e. MySQL longtext and longblob).
            """
            return False

        def prepare_default(self, value):
            """
            Only used for backends which have requires_literal_defaults feature
            """
            raise NotImplementedError(
                'subclasses of BaseDatabaseSchemaEditor for backends which have '
                'requires_literal_defaults must provide a prepare_default() method'
            )

        def effective_default(self, field):
            """Return a field's effective database default value."""
            if field.has_default():
                default = field.get_default()
            elif not field.null and field.blank and field.empty_strings_allowed:
                if field.get_internal_type() == "BinaryField":
                    default = bytes()
                else:
                    default = str()
            elif getattr(field, 'auto_now', False) or getattr(field, 'auto_now_add', False):
                default = datetime.now()
                internal_type = field.get_internal_type()
                if internal_type == 'DateField':
                    default = default.date
                elif internal_type == 'TimeField':
                    default = default.time
                elif internal_type == 'DateTimeField':
                    default = timezone.now
            else:
                default = None
            # If it's a callable, call it
            if callable(default):
                default = default()
            # Run it through the field's get_db_prep_save method so we can send it
            # to the database.
            default = field.get_db_prep_save(default, self.connection)
            return default

        def quote_value(self, value):
            """
            Return a quoted version of the value so it's safe to use in an SQL
            string. This is not safe against injection from user code; it is
            intended only for use in making SQL scripts or preparing default values
            for particularly tricky backends (defaults are not user-defined, though,
            so this is safe).
            """
            raise NotImplementedError()

        # Actions

        def create_model(self, model):
            """
            Create a table and any accompanying indexes or unique constraints for
            the given `model`.
            """
            # Create column SQL, add FK deferreds if needed
            column_sqls = []
            params = []
            for field in model._meta.local_fields:
                # SQL
                definition, extra_params = self.column_sql(model, field)
                if definition is None:
                    continue
                # Check constraints can go on the column SQL here
                db_params = field.db_parameters(connection=self.connection)
                if db_params['check']:
                    definition += " CHECK (%s)" % db_params['check']
                # Autoincrement SQL (for backends with inline variant)
                col_type_suffix = field.db_type_suffix(connection=self.connection)
                if col_type_suffix:
                    definition += " %s" % col_type_suffix
                params.extend(extra_params)
                # FK
                if field.remote_field and field.db_constraint:
                    to_table = field.remote_field.model._meta.db_table
                    to_column = field.remote_field.model._meta.get_field(field.remote_field.field_name).column
                    if self.sql_create_inline_fk:
                        definition += " " + self.sql_create_inline_fk % {
                            "to_table": self.quote_name(to_table),
                            "to_column": self.quote_name(to_column),
                        }
                    elif self.connection.features.supports_foreign_keys:
                        self.deferred_sql.append(self._create_fk_sql(model, field, "_fk_%(to_table)s_%(to_column)s"))
                # Add the SQL to our big list
                column_sqls.append("%s %s" % (
                    self.quote_name(field.column),
                    definition,
                ))
                # Autoincrement SQL (for backends with post table definition variant)
                if field.get_internal_type() in ("AutoField", "BigAutoField"):
                    autoinc_sql = self.connection.ops.autoinc_sql(model._meta.db_table, field.column)
                    if autoinc_sql:
                        self.deferred_sql.extend(autoinc_sql)

            # Add any unique_togethers (always deferred, as some fields might be
            # created afterwards, like geometry fields with some backends)
            for fields in model._meta.unique_together:
                columns = [model._meta.get_field(field).column for field in fields]
                self.deferred_sql.append(self._create_unique_sql(model, columns))
            # Make the table
            sql = self.sql_create_table % {
                "table": self.quote_name(model._meta.db_table),
                "definition": ", ".join(column_sqls)
            }
            if model._meta.db_tablespace:
                tablespace_sql = self.connection.ops.tablespace_sql(model._meta.db_tablespace)
                if tablespace_sql:
                    sql += ' ' + tablespace_sql
            # Prevent using [] as params, in the case a literal '%' is used in the definition
            self.execute(sql, params or None)

            # Add any field index and index_together's (deferred as SQLite3 _remake_table needs it)
            self.deferred_sql.extend(self._model_indexes_sql(model))

            # Make M2M tables
            for field in model._meta.local_many_to_many:
                if field.remote_field.through._meta.auto_created:
                    self.create_model(field.remote_field.through)

        def delete_model(self, model):
            """Delete a model from the database."""
            # Handle auto-created intermediary models
            for field in model._meta.local_many_to_many:
                if field.remote_field.through._meta.auto_created:
                    self.delete_model(field.remote_field.through)

            # Delete the table
            self.execute(self.sql_delete_table % {
                "table": self.quote_name(model._meta.db_table),
            })
            # Remove all deferred statements referencing the deleted table.
            for sql in list(self.deferred_sql):
                if isinstance(sql, Statement) and sql.references_table(model._meta.db_table):
                    self.deferred_sql.remove(sql)

        def add_index(self, model, index):
            """Add an index on a model."""
            self.execute(index.create_sql(model, self))

        def remove_index(self, model, index):
            """Remove an index from a model."""
            self.execute(index.remove_sql(model, self))

        def alter_unique_together(self, model, old_unique_together, new_unique_together):
            """
            Deal with a model changing its unique_together. The input
            unique_togethers must be doubly-nested, not the single-nested
            ["foo", "bar"] format.
            """
            olds = {tuple(fields) for fields in old_unique_together}
            news = {tuple(fields) for fields in new_unique_together}
            # Deleted uniques
            for fields in olds.difference(news):
                self._delete_composed_index(model, fields, {'unique': True}, self.sql_delete_unique)
            # Created uniques
            for fields in news.difference(olds):
                columns = [model._meta.get_field(field).column for field in fields]
                self.execute(self._create_unique_sql(model, columns))

        def alter_index_together(self, model, old_index_together, new_index_together):
            """
            Deal with a model changing its index_together. The input
            index_togethers must be doubly-nested, not the single-nested
            ["foo", "bar"] format.
            """
            olds = {tuple(fields) for fields in old_index_together}
            news = {tuple(fields) for fields in new_index_together}
            # Deleted indexes
            for fields in olds.difference(news):
                self._delete_composed_index(model, fields, {'index': True}, self.sql_delete_index)
            # Created indexes
            for field_names in news.difference(olds):
                fields = [model._meta.get_field(field) for field in field_names]
                self.execute(self._create_index_sql(model, fields, suffix="_idx"))

        def _delete_composed_index(self, model, fields, constraint_kwargs, sql):
            columns = [model._meta.get_field(field).column for field in fields]
            constraint_names = self._constraint_names(model, columns, **constraint_kwargs)
            if len(constraint_names) != 1:
                raise ValueError("Found wrong number (%s) of constraints for %s(%s)" % (
                    len(constraint_names),
                    model._meta.db_table,
                    ", ".join(columns),
                ))
            self.execute(self._delete_constraint_sql(sql, model, constraint_names[0]))

        def alter_db_table(self, model, old_db_table, new_db_table):
            """Rename the table a model points to."""
            if (old_db_table == new_db_table or
                (self.connection.features.ignores_table_name_case and
                    old_db_table.lower() == new_db_table.lower())):
                return
            self.execute(self.sql_rename_table % {
                "old_table": self.quote_name(old_db_table),
                "new_table": self.quote_name(new_db_table),
            })
            # Rename all references to the old table name.
            for sql in self.deferred_sql:
                if isinstance(sql, Statement):
                    sql.rename_table_references(old_db_table, new_db_table)

        def alter_db_tablespace(self, model, old_db_tablespace, new_db_tablespace):
            """Move a model's table between tablespaces."""
            self.execute(self.sql_retablespace_table % {
                "table": self.quote_name(model._meta.db_table),
                "old_tablespace": self.quote_name(old_db_tablespace),
                "new_tablespace": self.quote_name(new_db_tablespace),
            })

        def add_field(self, model, field):
            """
            Create a field on a model. Usually involves adding a column, but may
            involve adding a table instead (for M2M fields).
            """
            # Special-case implicit M2M tables
            if field.many_to_many and field.remote_field.through._meta.auto_created:
                return self.create_model(field.remote_field.through)
            # Get the column's definition
            definition, params = self.column_sql(model, field, include_default=True)
            # It might not actually have a column behind it
            if definition is None:
                return
            # Check constraints can go on the column SQL here
            db_params = field.db_parameters(connection=self.connection)
            if db_params['check']:
                definition += " CHECK (%s)" % db_params['check']
            # Build the SQL and run it
            sql = self.sql_create_column % {
                "table": self.quote_name(model._meta.db_table),
                "column": self.quote_name(field.column),
                "definition": definition,
            }
            self.execute(sql, params)
            # Drop the default if we need to
            # (Django usually does not use in-database defaults)
            if not self.skip_default(field) and self.effective_default(field) is not None:
                changes_sql, params = self._alter_column_default_sql(model, None, field, drop=True)
                sql = self.sql_alter_column % {
                    "table": self.quote_name(model._meta.db_table),
                    "changes": changes_sql,
                }
                self.execute(sql, params)
            # Add an index, if required
            self.deferred_sql.extend(self._field_indexes_sql(model, field))
            # Add any FK constraints later
            if field.remote_field and self.connection.features.supports_foreign_keys and field.db_constraint:
                self.deferred_sql.append(self._create_fk_sql(model, field, "_fk_%(to_table)s_%(to_column)s"))
            # Reset connection if required
            if self.connection.features.connection_persists_old_columns:
                self.connection.close()

        def remove_field(self, model, field):
            """
            Remove a field from a model. Usually involves deleting a column,
            but for M2Ms may involve deleting a table.
            """
            # Special-case implicit M2M tables
            if field.many_to_many and field.remote_field.through._meta.auto_created:
                return self.delete_model(field.remote_field.through)
            # It might not actually have a column behind it
            if field.db_parameters(connection=self.connection)['type'] is None:
                return
            # Drop any FK constraints, MySQL requires explicit deletion
            if field.remote_field:
                fk_names = self._constraint_names(model, [field.column], foreign_key=True)
                for fk_name in fk_names:
                    self.execute(self._delete_constraint_sql(self.sql_delete_fk, model, fk_name))
            # Delete the column
            sql = self.sql_delete_column % {
                "table": self.quote_name(model._meta.db_table),
                "column": self.quote_name(field.column),
            }
            self.execute(sql)
            # Reset connection if required
            if self.connection.features.connection_persists_old_columns:
                self.connection.close()
            # Remove all deferred statements referencing the deleted table.
            for sql in list(self.deferred_sql):
                if isinstance(sql, Statement) and sql.references_column(model._meta.db_table, field.column):
                    self.deferred_sql.remove(sql)

        def alter_field(self, model, old_field, new_field, strict=False):
            """
            Allow a field's type, uniqueness, nullability, default, column,
            constraints, etc. to be modified.
            `old_field` is required to compute the necessary changes.
            If `strict` is True, raise errors if the old column does not match
            `old_field` precisely.
            """
            # Ensure this field is even column-based
            old_db_params = old_field.db_parameters(connection=self.connection)
            old_type = old_db_params['type']
            new_db_params = new_field.db_parameters(connection=self.connection)
            new_type = new_db_params['type']
            if ((old_type is None and old_field.remote_field is None) or
                    (new_type is None and new_field.remote_field is None)):
                raise ValueError(
                    "Cannot alter field %s into %s - they do not properly define "
                    "db_type (are you using a badly-written custom field?)" %
                    (old_field, new_field),
                )
            elif old_type is None and new_type is None and (
                    old_field.remote_field.through and new_field.remote_field.through and
                    old_field.remote_field.through._meta.auto_created and
                    new_field.remote_field.through._meta.auto_created):
                return self._alter_many_to_many(model, old_field, new_field, strict)
            elif old_type is None and new_type is None and (
                    old_field.remote_field.through and new_field.remote_field.through and
                    not old_field.remote_field.through._meta.auto_created and
                    not new_field.remote_field.through._meta.auto_created):
                # Both sides have through models; this is a no-op.
                return
            elif old_type is None or new_type is None:
                raise ValueError(
                    "Cannot alter field %s into %s - they are not compatible types "
                    "(you cannot alter to or from M2M fields, or add or remove "
                    "through= on M2M fields)" % (old_field, new_field)
                )

            self._alter_field(model, old_field, new_field, old_type, new_type,
                              old_db_params, new_db_params, strict)

        def _alter_field(self, model, old_field, new_field, old_type, new_type,
                         old_db_params, new_db_params, strict=False):
            """Perform a "physical" (non-ManyToMany) field update."""
            # Drop any FK constraints, we'll remake them later
            fks_dropped = set()
            if old_field.remote_field and old_field.db_constraint:
                fk_names = self._constraint_names(model, [old_field.column], foreign_key=True)
                if strict and len(fk_names) != 1:
                    raise ValueError("Found wrong number (%s) of foreign key constraints for %s.%s" % (
                        len(fk_names),
                        model._meta.db_table,
                        old_field.column,
                    ))
                for fk_name in fk_names:
                    fks_dropped.add((old_field.column,))
                    self.execute(self._delete_constraint_sql(self.sql_delete_fk, model, fk_name))
            # Has unique been removed?
            if old_field.unique and (not new_field.unique or (not old_field.primary_key and new_field.primary_key)):
                # Find the unique constraint for this field
                constraint_names = self._constraint_names(model, [old_field.column], unique=True)
                if strict and len(constraint_names) != 1:
                    raise ValueError("Found wrong number (%s) of unique constraints for %s.%s" % (
                        len(constraint_names),
                        model._meta.db_table,
                        old_field.column,
                    ))
                for constraint_name in constraint_names:
                    self.execute(self._delete_constraint_sql(self.sql_delete_unique, model, constraint_name))
            # Drop incoming FK constraints if the field is a primary key or unique,
            # which might be a to_field target, and things are going to change.
            drop_foreign_keys = (
                (
                    (old_field.primary_key and new_field.primary_key) or
                    (old_field.unique and new_field.unique)
                ) and old_type != new_type
            )
            if drop_foreign_keys:
                # '_meta.related_field' also contains M2M reverse fields, these
                # will be filtered out
                for _old_rel, new_rel in _related_non_m2m_objects(old_field, new_field):
                    rel_fk_names = self._constraint_names(
                        new_rel.related_model, [new_rel.field.column], foreign_key=True
                    )
                    for fk_name in rel_fk_names:
                        self.execute(self._delete_constraint_sql(self.sql_delete_fk, new_rel.related_model, fk_name))
            # Removed an index? (no strict check, as multiple indexes are possible)
            # Remove indexes if db_index switched to False or a unique constraint
            # will now be used in lieu of an index. The following lines from the
            # truth table show all True cases; the rest are False:
            #
            # old_field.db_index | old_field.unique | new_field.db_index | new_field.unique
            # ------------------------------------------------------------------------------
            # True               | False            | False              | False
            # True               | False            | False              | True
            # True               | False            | True               | True
            if old_field.db_index and not old_field.unique and (not new_field.db_index or new_field.unique):
                # Find the index for this field
                meta_index_names = {index.name for index in model._meta.indexes}
                # Retrieve only BTREE indexes since this is what's created with
                # db_index=True.
                index_names = self._constraint_names(model, [old_field.column], index=True, type_=Index.suffix)
                for index_name in index_names:
                    if index_name in meta_index_names:
                        # The only way to check if an index was created with
                        # db_index=True or with Index(['field'], name='foo')
                        # is to look at its name (refs #28053).
                        continue
                    self.execute(self._delete_constraint_sql(self.sql_delete_index, model, index_name))
            # Change check constraints?
            if old_db_params['check'] != new_db_params['check'] and old_db_params['check']:
                constraint_names = self._constraint_names(model, [old_field.column], check=True)
                if strict and len(constraint_names) != 1:
                    raise ValueError("Found wrong number (%s) of check constraints for %s.%s" % (
                        len(constraint_names),
                        model._meta.db_table,
                        old_field.column,
                    ))
                for constraint_name in constraint_names:
                    self.execute(self._delete_constraint_sql(self.sql_delete_check, model, constraint_name))
            # Have they renamed the column?
            if old_field.column != new_field.column:
                self.execute(self._rename_field_sql(model._meta.db_table, old_field, new_field, new_type))
                # Rename all references to the renamed column.
                for sql in self.deferred_sql:
                    if isinstance(sql, Statement):
                        sql.rename_column_references(model._meta.db_table, old_field.column, new_field.column)
            # Next, start accumulating actions to do
            actions = []
            null_actions = []
            post_actions = []
            # Type change?
            if old_type != new_type:
                fragment, other_actions = self._alter_column_type_sql(model, old_field, new_field, new_type)
                actions.append(fragment)
                post_actions.extend(other_actions)
            # When changing a column NULL constraint to NOT NULL with a given
            # default value, we need to perform 4 steps:
            #  1. Add a default for new incoming writes
            #  2. Update existing NULL rows with new default
            #  3. Replace NULL constraint with NOT NULL
            #  4. Drop the default again.
            # Default change?
            old_default = self.effective_default(old_field)
            new_default = self.effective_default(new_field)
            needs_database_default = (
                old_field.null and
                not new_field.null and
                old_default != new_default and
                new_default is not None and
                not self.skip_default(new_field)
            )
            if needs_database_default:
                actions.append(self._alter_column_default_sql(model, old_field, new_field))
            # Nullability change?
            if old_field.null != new_field.null:
                fragment = self._alter_column_null_sql(model, old_field, new_field)
                if fragment:
                    null_actions.append(fragment)
            # Only if we have a default and there is a change from NULL to NOT NULL
            four_way_default_alteration = (
                new_field.has_default() and
                (old_field.null and not new_field.null)
            )
            if actions or null_actions:
                if not four_way_default_alteration:
                    # If we don't have to do a 4-way default alteration we can
                    # directly run a (NOT) NULL alteration
                    actions = actions + null_actions
                # Combine actions together if we can (e.g. postgres)
                if self.connection.features.supports_combined_alters and actions:
                    sql, params = tuple(zip(*actions))
                    actions = [(", ".join(sql), sum(params, []))]
                # Apply those actions
                for sql, params in actions:
                    self.execute(
                        self.sql_alter_column % {
                            "table": self.quote_name(model._meta.db_table),
                            "changes": sql,
                        },
                        params,
                    )
                if four_way_default_alteration:
                    # Update existing rows with default value
                    self.execute(
                        self.sql_update_with_default % {
                            "table": self.quote_name(model._meta.db_table),
                            "column": self.quote_name(new_field.column),
                            "default": "%s",
                        },
                        [new_default],
                    )
                    # Since we didn't run a NOT NULL change before we need to do it
                    # now
                    for sql, params in null_actions:
                        self.execute(
                            self.sql_alter_column % {
                                "table": self.quote_name(model._meta.db_table),
                                "changes": sql,
                            },
                            params,
                        )
            if post_actions:
                for sql, params in post_actions:
                    self.execute(sql, params)
            # Added a unique?
            if (not old_field.unique and new_field.unique) or (
                old_field.primary_key and not new_field.primary_key and new_field.unique
            ):
                self.execute(self._create_unique_sql(model, [new_field.column]))
            # Added an index? Add an index if db_index switched to True or a unique
            # constraint will no longer be used in lieu of an index. The following
            # lines from the truth table show all True cases; the rest are False:
            #
            # old_field.db_index | old_field.unique | new_field.db_index | new_field.unique
            # ------------------------------------------------------------------------------
            # False              | False            | True               | False
            # False              | True             | True               | False
            # True               | True             | True               | False
            if (not old_field.db_index or old_field.unique) and new_field.db_index and not new_field.unique:
                self.execute(self._create_index_sql(model, [new_field]))
            # Type alteration on primary key? Then we need to alter the column
            # referring to us.
            rels_to_update = []
            if old_field.primary_key and new_field.primary_key and old_type != new_type:
                rels_to_update.extend(_related_non_m2m_objects(old_field, new_field))
            # Changed to become primary key?
            # Note that we don't detect unsetting of a PK, as we assume another field
            # will always come along and replace it.
            if not old_field.primary_key and new_field.primary_key:
                # First, drop the old PK
                self._delete_primary_key(model, strict)
                # Make the new one
                self.execute(
                    self.sql_create_pk % {
                        "table": self.quote_name(model._meta.db_table),
                        "name": self.quote_name(
                            self._create_index_name(model._meta.db_table, [new_field.column], suffix="_pk")
                        ),
                        "columns": self.quote_name(new_field.column),
                    }
                )
                # Update all referencing columns
                rels_to_update.extend(_related_non_m2m_objects(old_field, new_field))
            # Handle our type alters on the other end of rels from the PK stuff above
            for old_rel, new_rel in rels_to_update:
                rel_db_params = new_rel.field.db_parameters(connection=self.connection)
                rel_type = rel_db_params['type']
                fragment, other_actions = self._alter_column_type_sql(
                    new_rel.related_model, old_rel.field, new_rel.field, rel_type
                )
                self.execute(
                    self.sql_alter_column % {
                        "table": self.quote_name(new_rel.related_model._meta.db_table),
                        "changes": fragment[0],
                    },
                    fragment[1],
                )
                for sql, params in other_actions:
                    self.execute(sql, params)
            # Does it have a foreign key?
            if (new_field.remote_field and
                    (fks_dropped or not old_field.remote_field or not old_field.db_constraint) and
                    new_field.db_constraint):
                self.execute(self._create_fk_sql(model, new_field, "_fk_%(to_table)s_%(to_column)s"))
            # Rebuild FKs that pointed to us if we previously had to drop them
            if drop_foreign_keys:
                for rel in new_field.model._meta.related_objects:
                    if _is_relevant_relation(rel, new_field) and rel.field.db_constraint:
                        self.execute(self._create_fk_sql(rel.related_model, rel.field, "_fk"))
            # Does it have check constraints we need to add?
            if old_db_params['check'] != new_db_params['check'] and new_db_params['check']:
                self.execute(
                    self.sql_create_check % {
                        "table": self.quote_name(model._meta.db_table),
                        "name": self.quote_name(
                            self._create_index_name(model._meta.db_table, [new_field.column], suffix="_check")
                        ),
                        "column": self.quote_name(new_field.column),
                        "check": new_db_params['check'],
                    }
                )
            # Drop the default if we need to
            # (Django usually does not use in-database defaults)
            if needs_database_default:
                changes_sql, params = self._alter_column_default_sql(model, old_field, new_field, drop=True)
                sql = self.sql_alter_column % {
                    "table": self.quote_name(model._meta.db_table),
                    "changes": changes_sql,
                }
                self.execute(sql, params)
            # Reset connection if required
            if self.connection.features.connection_persists_old_columns:
                self.connection.close()

        def _alter_column_null_sql(self, model, old_field, new_field):
            """
            Hook to specialize column null alteration.

            Return a (sql, params) fragment to set a column to null or non-null
            as required by new_field, or None if no changes are required.
            """
            if (self.connection.features.interprets_empty_strings_as_nulls and
                    new_field.get_internal_type() in ("CharField", "TextField")):
                # The field is nullable in the database anyway, leave it alone.
                return
            else:
                new_db_params = new_field.db_parameters(connection=self.connection)
                sql = self.sql_alter_column_null if new_field.null else self.sql_alter_column_not_null
                return (
                    sql % {
                        'column': self.quote_name(new_field.column),
                        'type': new_db_params['type'],
                    },
                    [],
                )

        def _alter_column_default_sql(self, model, old_field, new_field, drop=False):
            """
            Hook to specialize column default alteration.

            Return a (sql, params) fragment to add or drop (depending on the drop
            argument) a default to new_field's column.
            """
            new_default = self.effective_default(new_field)
            default = '%s'
            params = [new_default]

            if drop:
                params = []
            elif self.connection.features.requires_literal_defaults:
                # Some databases (Oracle) can't take defaults as a parameter
                # If this is the case, the SchemaEditor for that database should
                # implement prepare_default().
                default = self.prepare_default(new_default)
                params = []

            new_db_params = new_field.db_parameters(connection=self.connection)
            sql = self.sql_alter_column_no_default if drop else self.sql_alter_column_default
            return (
                sql % {
                    'column': self.quote_name(new_field.column),
                    'type': new_db_params['type'],
                    'default': default,
                },
                params,
            )

        def _alter_column_type_sql(self, model, old_field, new_field, new_type):
            """
            Hook to specialize column type alteration for different backends,
            for cases when a creation type is different to an alteration type
            (e.g. SERIAL in PostgreSQL, PostGIS fields).

            Return a two-tuple of: an SQL fragment of (sql, params) to insert into
            an ALTER TABLE statement and a list of extra (sql, params) tuples to
            run once the field is altered.
            """
            return (
                (
                    self.sql_alter_column_type % {
                        "column": self.quote_name(new_field.column),
                        "type": new_type,
                    },
                    [],
                ),
                [],
            )

        def _alter_many_to_many(self, model, old_field, new_field, strict):
            """Alter M2Ms to repoint their to= endpoints."""
            # Rename the through table
            if old_field.remote_field.through._meta.db_table != new_field.remote_field.through._meta.db_table:
                self.alter_db_table(old_field.remote_field.through, old_field.remote_field.through._meta.db_table,
                                    new_field.remote_field.through._meta.db_table)
            # Repoint the FK to the other side
            self.alter_field(
                new_field.remote_field.through,
                # We need the field that points to the target model, so we can tell alter_field to change it -
                # this is m2m_reverse_field_name() (as opposed to m2m_field_name, which points to our model)
                old_field.remote_field.through._meta.get_field(old_field.m2m_reverse_field_name()),
                new_field.remote_field.through._meta.get_field(new_field.m2m_reverse_field_name()),
            )
            self.alter_field(
                new_field.remote_field.through,
                # for self-referential models we need to alter field from the other end too
                old_field.remote_field.through._meta.get_field(old_field.m2m_field_name()),
                new_field.remote_field.through._meta.get_field(new_field.m2m_field_name()),
            )

        def _create_index_name(self, table_name, column_names, suffix=""):
            """
            Generate a unique name for an index/unique constraint.

            The name is divided into 3 parts: the table name, the column names,
            and a unique digest and suffix.
            """
            _, table_name = split_identifier(table_name)
            hash_data = [table_name] + list(column_names)
            hash_suffix_part = '%s%s' % (self._digest(*hash_data), suffix)
            max_length = self.connection.ops.max_name_length() or 200
            # If everything fits into max_length, use that name.
            index_name = '%s_%s_%s' % (table_name, '_'.join(column_names), hash_suffix_part)
            if len(index_name) <= max_length:
                return index_name
            # Shorten a long suffix.
            if len(hash_suffix_part) > max_length / 3:
                hash_suffix_part = hash_suffix_part[:max_length // 3]
            other_length = (max_length - len(hash_suffix_part)) // 2 - 1
            index_name = '%s_%s_%s' % (
                table_name[:other_length],
                '_'.join(column_names)[:other_length],
                hash_suffix_part,
            )
            # Prepend D if needed to prevent the name from starting with an
            # underscore or a number (not permitted on Oracle).
            if index_name[0] == "_" or index_name[0].isdigit():
                index_name = "D%s" % index_name[:-1]
            return index_name

        def _get_index_tablespace_sql(self, model, fields, db_tablespace=None):
            if db_tablespace is None:
                if len(fields) == 1 and fields[0].db_tablespace:
                    db_tablespace = fields[0].db_tablespace
                elif model._meta.db_tablespace:
                    db_tablespace = model._meta.db_tablespace
            if db_tablespace is not None:
                return ' ' + self.connection.ops.tablespace_sql(db_tablespace)
            return ''

        def _create_index_sql(self, model, fields, *, name=None, suffix='', using='',
                              db_tablespace=None, col_suffixes=(), sql=None):
            """
            Return the SQL statement to create the index for one or several fields.
            `sql` can be specified if the syntax differs from the standard (GIS
            indexes, ...).
            """
            tablespace_sql = self._get_index_tablespace_sql(model, fields, db_tablespace=db_tablespace)
            columns = [field.column for field in fields]
            sql_create_index = sql or self.sql_create_index
            table = model._meta.db_table

            def create_index_name(*args, **kwargs):
                nonlocal name
                if name is None:
                    name = self._create_index_name(*args, **kwargs)
                return self.quote_name(name)

            return Statement(
                sql_create_index,
                table=Table(table, self.quote_name),
                name=IndexName(table, columns, suffix, create_index_name),
                using=using,
                columns=Columns(table, columns, self.quote_name, col_suffixes=col_suffixes),
                extra=tablespace_sql,
            )

        def _model_indexes_sql(self, model):
            """
            Return a list of all index SQL statements (field indexes,
            index_together, Meta.indexes) for the specified model.
            """
            if not model._meta.managed or model._meta.proxy or model._meta.swapped:
                return []
            output = []
            for field in model._meta.local_fields:
                output.extend(self._field_indexes_sql(model, field))

            for field_names in model._meta.index_together:
                fields = [model._meta.get_field(field) for field in field_names]
                output.append(self._create_index_sql(model, fields, suffix="_idx"))

            for index in model._meta.indexes:
                output.append(index.create_sql(model, self))
            return output

        def _field_indexes_sql(self, model, field):
            """
            Return a list of all index SQL statements for the specified field.
            """
            output = []
            if self._field_should_be_indexed(model, field):
                output.append(self._create_index_sql(model, [field]))
            return output

        def _field_should_be_indexed(self, model, field):
            return field.db_index and not field.unique

        def _rename_field_sql(self, table, old_field, new_field, new_type):
            return self.sql_rename_column % {
                "table": self.quote_name(table),
                "old_column": self.quote_name(old_field.column),
                "new_column": self.quote_name(new_field.column),
                "type": new_type,
            }

        def _create_fk_sql(self, model, field, suffix):
            from_table = model._meta.db_table
            from_column = field.column
            _, to_table = split_identifier(field.target_field.model._meta.db_table)
            to_column = field.target_field.column

            def create_fk_name(*args, **kwargs):
                return self.quote_name(self._create_index_name(*args, **kwargs))

            return Statement(
                self.sql_create_fk,
                table=Table(from_table, self.quote_name),
                name=ForeignKeyName(from_table, [from_column], to_table, [to_column], suffix, create_fk_name),
                column=Columns(from_table, [from_column], self.quote_name),
                to_table=Table(field.target_field.model._meta.db_table, self.quote_name),
                to_column=Columns(field.target_field.model._meta.db_table, [to_column], self.quote_name),
                deferrable=self.connection.ops.deferrable_sql(),
            )

        def _create_unique_sql(self, model, columns):
            table = model._meta.db_table
            return Statement(
                self.sql_create_unique,
                table=Table(table, self.quote_name),
                name=IndexName(table, columns, '_uniq', self._create_index_name),
                columns=Columns(table, columns, self.quote_name),
            )

        def _delete_constraint_sql(self, template, model, name):
            return template % {
                "table": self.quote_name(model._meta.db_table),
                "name": self.quote_name(name),
            }

        def _constraint_names(self, model, column_names=None, unique=None,
                              primary_key=None, index=None, foreign_key=None,
                              check=None, type_=None):
            """Return all constraint names matching the columns and conditions."""
            if column_names is not None:
                column_names = [
                    self.connection.introspection.column_name_converter(name)
                    for name in column_names
                ]
            with self.connection.cursor() as cursor:
                constraints = self.connection.introspection.get_constraints(cursor, model._meta.db_table)
            result = []
            for name, infodict in constraints.items():
                if column_names is None or column_names == infodict['columns']:
                    if unique is not None and infodict['unique'] != unique:
                        continue
                    if primary_key is not None and infodict['primary_key'] != primary_key:
                        continue
                    if index is not None and infodict['index'] != index:
                        continue
                    if check is not None and infodict['check'] != check:
                        continue
                    if foreign_key is not None and not infodict['foreign_key']:
                        continue
                    if type_ is not None and infodict['type'] != type_:
                        continue
                    result.append(name)
            return result

        def _delete_primary_key(self, model, strict=False):
            constraint_names = self._constraint_names(model, primary_key=True)
            if strict and len(constraint_names) != 1:
                raise ValueError('Found wrong number (%s) of PK constraints for %s' % (
                    len(constraint_names),
                    model._meta.db_table,
                ))
            for constraint_name in constraint_names:
                self.execute(self._delete_constraint_sql(self.sql_delete_pk, model, constraint_name))

        def remove_procedure(self, procedure_name, param_types=()):
            sql = self.sql_delete_procedure % {
                'procedure': self.quote_name(procedure_name),
                'param_types': ','.join(param_types),
            }
            self.execute(sql)