[Answered ]-How do migration operations (eg. AddField) detect if you're applying or reverting a migration?

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The migrate management command will detect if this is a new (i.e. forward) or previous (i.e. backward) migration, and call either the database_forwards or database_backwards method depending on the direction of the migration. The Operation subclass doesn’t need to know the direction on init:

class AddField(FieldOperation):
    ...
    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)
            )

This is partly explained in the documentation, specifically in reversing migrations and writing your own migration operations sections.

👤Selcuk

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