# documentación de Django - 1.10.7

# Código fuente para django.db.migrations.operations.fields

    from __future__ import unicode_literals

    from django.db.models.fields import NOT_PROVIDED
    from django.utils.functional import cached_property

    from .base import Operation

    class FieldOperation(Operation):
        def __init__(self, model_name, name):
            self.model_name = model_name
            self.name = name

        @cached_property
        def model_name_lower(self):
            return self.model_name.lower()

        @cached_property
        def name_lower(self):
            return self.name.lower()

        def is_same_model_operation(self, operation):
            return self.model_name_lower == operation.model_name_lower

        def is_same_field_operation(self, operation):
            return self.is_same_model_operation(operation) and self.name_lower == operation.name_lower

        def references_model(self, name, app_label=None):
            return name.lower() == self.model_name_lower

        def references_field(self, model_name, name, app_label=None):
            return self.references_model(model_name) and name.lower() == self.name_lower

        def reduce(self, operation, in_between, app_label=None):
            return (
                super(FieldOperation, self).reduce(operation, in_between, app_label=app_label) or
                not operation.references_field(self.model_name, self.name, app_label)
            )

    class AddField(FieldOperation):
        """
        Adds a field to a model.
        """

        def __init__(self, model_name, name, field, preserve_default=True):
            self.field = field
            self.preserve_default = preserve_default
            super(AddField, self).__init__(model_name, name)

        def deconstruct(self):
            kwargs = {
                'model_name': self.model_name,
                'name': self.name,
                'field': self.field,
            }
            if self.preserve_default is not True:
                kwargs['preserve_default'] = self.preserve_default
            return (
                self.__class__.__name__,
                [],
                kwargs
            )

        def state_forwards(self, app_label, state):
            # If preserve default is off, don't use the default for future state
            if not self.preserve_default:
                field = self.field.clone()
                field.default = NOT_PROVIDED
            else:
                field = self.field
            state.models[app_label, self.model_name_lower].fields.append((self.name, field))
            state.reload_model(app_label, self.model_name_lower)

        def database_forwards(self, app_label, schema_editor, from_state, to_state):
            to_model = to_state.apps.get_model(app_label, self.model_name)
            if self.allow_migrate_model(schema_editor.connection.alias, to_model):
                from_model = from_state.apps.get_model(app_label, self.model_name)
                field = to_model._meta.get_field(self.name)
                if not self.preserve_default:
                    field.default = self.field.default
                schema_editor.add_field(
                    from_model,
                    field,
                )
                if not self.preserve_default:
                    field.default = NOT_PROVIDED

        def database_backwards(self, app_label, schema_editor, from_state, to_state):
            from_model = from_state.apps.get_model(app_label, self.model_name)
            if self.allow_migrate_model(schema_editor.connection.alias, from_model):
                schema_editor.remove_field(from_model, from_model._meta.get_field(self.name))

        def describe(self):
            return "Add field %s to %s" % (self.name, self.model_name)

        def reduce(self, operation, in_between, app_label=None):
            if isinstance(operation, FieldOperation) and self.is_same_field_operation(operation):
                if isinstance(operation, AlterField):
                    return [
                        AddField(
                            model_name=self.model_name,
                            name=operation.name,
                            field=operation.field,
                        ),
                    ]
                elif isinstance(operation, RemoveField):
                    return []
                elif isinstance(operation, RenameField):
                    return [
                        AddField(
                            model_name=self.model_name,
                            name=operation.new_name,
                            field=self.field,
                        ),
                    ]
            return super(AddField, self).reduce(operation, in_between, app_label=app_label)

    class RemoveField(FieldOperation):
        """
        Removes a field from a model.
        """

        def deconstruct(self):
            kwargs = {
                'model_name': self.model_name,
                'name': self.name,
            }
            return (
                self.__class__.__name__,
                [],
                kwargs
            )

        def state_forwards(self, app_label, state):
            new_fields = []
            for name, instance in state.models[app_label, self.model_name_lower].fields:
                if name != self.name:
                    new_fields.append((name, instance))
            state.models[app_label, self.model_name_lower].fields = new_fields
            state.reload_model(app_label, self.model_name_lower)

        def database_forwards(self, app_label, schema_editor, from_state, to_state):
            from_model = from_state.apps.get_model(app_label, self.model_name)
            if self.allow_migrate_model(schema_editor.connection.alias, from_model):
                schema_editor.remove_field(from_model, from_model._meta.get_field(self.name))

        def database_backwards(self, app_label, schema_editor, from_state, to_state):
            to_model = to_state.apps.get_model(app_label, self.model_name)
            if self.allow_migrate_model(schema_editor.connection.alias, to_model):
                from_model = from_state.apps.get_model(app_label, self.model_name)
                schema_editor.add_field(from_model, to_model._meta.get_field(self.name))

        def describe(self):
            return "Remove field %s from %s" % (self.name, self.model_name)

    class AlterField(FieldOperation):
        """
        Alters a field's database column (e.g. null, max_length) to the provided new field
        """

        def __init__(self, model_name, name, field, preserve_default=True):
            self.field = field
            self.preserve_default = preserve_default
            super(AlterField, self).__init__(model_name, name)

        def deconstruct(self):
            kwargs = {
                'model_name': self.model_name,
                'name': self.name,
                'field': self.field,
            }
            if self.preserve_default is not True:
                kwargs['preserve_default'] = self.preserve_default
            return (
                self.__class__.__name__,
                [],
                kwargs
            )

        def state_forwards(self, app_label, state):
            if not self.preserve_default:
                field = self.field.clone()
                field.default = NOT_PROVIDED
            else:
                field = self.field
            state.models[app_label, self.model_name_lower].fields = [
                (n, field if n == self.name else f)
                for n, f in
                state.models[app_label, self.model_name_lower].fields
            ]
            state.reload_model(app_label, self.model_name_lower)

        def database_forwards(self, app_label, schema_editor, from_state, to_state):
            to_model = to_state.apps.get_model(app_label, self.model_name)
            if self.allow_migrate_model(schema_editor.connection.alias, to_model):
                from_model = from_state.apps.get_model(app_label, self.model_name)
                from_field = from_model._meta.get_field(self.name)
                to_field = to_model._meta.get_field(self.name)
                if not self.preserve_default:
                    to_field.default = self.field.default
                schema_editor.alter_field(from_model, from_field, to_field)
                if not self.preserve_default:
                    to_field.default = NOT_PROVIDED

        def database_backwards(self, app_label, schema_editor, from_state, to_state):
            self.database_forwards(app_label, schema_editor, from_state, to_state)

        def describe(self):
            return "Alter field %s on %s" % (self.name, self.model_name)

        def reduce(self, operation, in_between, app_label=None):
            if isinstance(operation, RemoveField) and self.is_same_field_operation(operation):
                return [operation]
            elif isinstance(operation, RenameField) and self.is_same_field_operation(operation):
                return [
                    operation,
                    AlterField(
                        model_name=self.model_name,
                        name=operation.new_name,
                        field=self.field,
                    ),
                ]
            return super(AlterField, self).reduce(operation, in_between, app_label=app_label)

    class RenameField(FieldOperation):
        """
        Renames a field on the model. Might affect db_column too.
        """

        def __init__(self, model_name, old_name, new_name):
            self.old_name = old_name
            self.new_name = new_name
            super(RenameField, self).__init__(model_name, old_name)

        @cached_property
        def old_name_lower(self):
            return self.old_name.lower()

        @cached_property
        def new_name_lower(self):
            return self.new_name.lower()

        def deconstruct(self):
            kwargs = {
                'model_name': self.model_name,
                'old_name': self.old_name,
                'new_name': self.new_name,
            }
            return (
                self.__class__.__name__,
                [],
                kwargs
            )

        def state_forwards(self, app_label, state):
            # Rename the field
            state.models[app_label, self.model_name_lower].fields = [
                (self.new_name if n == self.old_name else n, f)
                for n, f in state.models[app_label, self.model_name_lower].fields
            ]
            # Fix index/unique_together to refer to the new field
            options = state.models[app_label, self.model_name_lower].options
            for option in ('index_together', 'unique_together'):
                if option in options:
                    options[option] = [
                        [self.new_name if n == self.old_name else n for n in together]
                        for together in options[option]
                    ]
            state.reload_model(app_label, self.model_name_lower)

        def database_forwards(self, app_label, schema_editor, from_state, to_state):
            to_model = to_state.apps.get_model(app_label, self.model_name)
            if self.allow_migrate_model(schema_editor.connection.alias, to_model):
                from_model = from_state.apps.get_model(app_label, self.model_name)
                schema_editor.alter_field(
                    from_model,
                    from_model._meta.get_field(self.old_name),
                    to_model._meta.get_field(self.new_name),
                )

        def database_backwards(self, app_label, schema_editor, from_state, to_state):
            to_model = to_state.apps.get_model(app_label, self.model_name)
            if self.allow_migrate_model(schema_editor.connection.alias, to_model):
                from_model = from_state.apps.get_model(app_label, self.model_name)
                schema_editor.alter_field(
                    from_model,
                    from_model._meta.get_field(self.new_name),
                    to_model._meta.get_field(self.old_name),
                )

        def describe(self):
            return "Rename field %s on %s to %s" % (self.old_name, self.model_name, self.new_name)

        def references_field(self, model_name, name, app_label=None):
            return self.references_model(model_name) and (
                name.lower() == self.old_name_lower or
                name.lower() == self.new_name_lower
            )

        def reduce(self, operation, in_between, app_label=None):
            if (isinstance(operation, RenameField) and
                    self.is_same_model_operation(operation) and
                    self.new_name_lower == operation.old_name_lower):
                return [
                    RenameField(
                        self.model_name,
                        self.old_name,
                        operation.new_name,
                    ),
                ]
            # Skip `FieldOperation.reduce` as we want to run `references_field`
            # against self.new_name.
            return (
                super(FieldOperation, self).reduce(operation, in_between, app_label=app_label) or
                not operation.references_field(self.model_name, self.new_name, app_label)
            )