# Τεκμηρίωση του Django - 1.10.7

# Πηγαίος κώδικας για το django.db.migrations.operations.special

    from __future__ import unicode_literals

    from django.db import router

    from .base import Operation

    class SeparateDatabaseAndState(Operation):
        """
        Takes two lists of operations - ones that will be used for the database,
        and ones that will be used for the state change. This allows operations
        that don't support state change to have it applied, or have operations
        that affect the state or not the database, or so on.
        """

        serialization_expand_args = ['database_operations', 'state_operations']

        def __init__(self, database_operations=None, state_operations=None):
            self.database_operations = database_operations or []
            self.state_operations = state_operations or []

        def deconstruct(self):
            kwargs = {}
            if self.database_operations:
                kwargs['database_operations'] = self.database_operations
            if self.state_operations:
                kwargs['state_operations'] = self.state_operations
            return (
                self.__class__.__name__,
                [],
                kwargs
            )

        def state_forwards(self, app_label, state):
            for state_operation in self.state_operations:
                state_operation.state_forwards(app_label, state)

        def database_forwards(self, app_label, schema_editor, from_state, to_state):
            # We calculate state separately in here since our state functions aren't useful
            for database_operation in self.database_operations:
                to_state = from_state.clone()
                database_operation.state_forwards(app_label, to_state)
                database_operation.database_forwards(app_label, schema_editor, from_state, to_state)
                from_state = to_state

        def database_backwards(self, app_label, schema_editor, from_state, to_state):
            # We calculate state separately in here since our state functions aren't useful
            to_states = {}
            for dbop in self.database_operations:
                to_states[dbop] = to_state
                to_state = to_state.clone()
                dbop.state_forwards(app_label, to_state)
            # to_state now has the states of all the database_operations applied
            # which is the from_state for the backwards migration of the last
            # operation.
            for database_operation in reversed(self.database_operations):
                from_state = to_state
                to_state = to_states[database_operation]
                database_operation.database_backwards(app_label, schema_editor, from_state, to_state)

        def describe(self):
            return "Custom state/database change combination"

    class RunSQL(Operation):
        """
        Runs some raw SQL. A reverse SQL statement may be provided.

        Also accepts a list of operations that represent the state change effected
        by this SQL change, in case it's custom column/table creation/deletion.
        """
        noop = ''

        def __init__(self, sql, reverse_sql=None, state_operations=None, hints=None, elidable=False):
            self.sql = sql
            self.reverse_sql = reverse_sql
            self.state_operations = state_operations or []
            self.hints = hints or {}
            self.elidable = elidable

        def deconstruct(self):
            kwargs = {
                'sql': self.sql,
            }
            if self.reverse_sql is not None:
                kwargs['reverse_sql'] = self.reverse_sql
            if self.state_operations:
                kwargs['state_operations'] = self.state_operations
            if self.hints:
                kwargs['hints'] = self.hints
            return (
                self.__class__.__name__,
                [],
                kwargs
            )

        @property
        def reversible(self):
            return self.reverse_sql is not None

        def state_forwards(self, app_label, state):
            for state_operation in self.state_operations:
                state_operation.state_forwards(app_label, state)

        def database_forwards(self, app_label, schema_editor, from_state, to_state):
            if router.allow_migrate(schema_editor.connection.alias, app_label, **self.hints):
                self._run_sql(schema_editor, self.sql)

        def database_backwards(self, app_label, schema_editor, from_state, to_state):
            if self.reverse_sql is None:
                raise NotImplementedError("You cannot reverse this operation")
            if router.allow_migrate(schema_editor.connection.alias, app_label, **self.hints):
                self._run_sql(schema_editor, self.reverse_sql)

        def describe(self):
            return "Raw SQL operation"

        def _run_sql(self, schema_editor, sqls):
            if isinstance(sqls, (list, tuple)):
                for sql in sqls:
                    params = None
                    if isinstance(sql, (list, tuple)):
                        elements = len(sql)
                        if elements == 2:
                            sql, params = sql
                        else:
                            raise ValueError("Expected a 2-tuple but got %d" % elements)
                    schema_editor.execute(sql, params=params)
            elif sqls != RunSQL.noop:
                statements = schema_editor.connection.ops.prepare_sql_script(sqls)
                for statement in statements:
                    schema_editor.execute(statement, params=None)

    class RunPython(Operation):
        """
        Runs Python code in a context suitable for doing versioned ORM operations.
        """

        reduces_to_sql = False

        def __init__(self, code, reverse_code=None, atomic=None, hints=None, elidable=False):
            self.atomic = atomic
            # Forwards code
            if not callable(code):
                raise ValueError("RunPython must be supplied with a callable")
            self.code = code
            # Reverse code
            if reverse_code is None:
                self.reverse_code = None
            else:
                if not callable(reverse_code):
                    raise ValueError("RunPython must be supplied with callable arguments")
                self.reverse_code = reverse_code
            self.hints = hints or {}
            self.elidable = elidable

        def deconstruct(self):
            kwargs = {
                'code': self.code,
            }
            if self.reverse_code is not None:
                kwargs['reverse_code'] = self.reverse_code
            if self.atomic is not None:
                kwargs['atomic'] = self.atomic
            if self.hints:
                kwargs['hints'] = self.hints
            return (
                self.__class__.__name__,
                [],
                kwargs
            )

        @property
        def reversible(self):
            return self.reverse_code is not None

        def state_forwards(self, app_label, state):
            # RunPython objects have no state effect. To add some, combine this
            # with SeparateDatabaseAndState.
            pass

        def database_forwards(self, app_label, schema_editor, from_state, to_state):
            if router.allow_migrate(schema_editor.connection.alias, app_label, **self.hints):
                # We now execute the Python code in a context that contains a 'models'
                # object, representing the versioned models as an app registry.
                # We could try to override the global cache, but then people will still
                # use direct imports, so we go with a documentation approach instead.
                self.code(from_state.apps, schema_editor)

        def database_backwards(self, app_label, schema_editor, from_state, to_state):
            if self.reverse_code is None:
                raise NotImplementedError("You cannot reverse this operation")
            if router.allow_migrate(schema_editor.connection.alias, app_label, **self.hints):
                self.reverse_code(from_state.apps, schema_editor)

        def describe(self):
            return "Raw Python operation"

        @staticmethod
        def noop(apps, schema_editor):
            return None