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

Question

A good example is django.db.migrations.AddField.

Let's say I've created a simple migration such as:

from django.db import migrations, models


class Migration(migrations.Migration):
    dependencies = []

    operations = [
        migrations.AddField(
            model_name="foo",
            name="bar",
            field=models.TextField(blank=True, null=True),
        ),
    ]

Running the migration would result in the bar field being added, and reverting to a previous migration would result in the bar field being removed.

However when I look under the hood of the AddField class, I don't see the logic for detecting the direction of the applied migration.

class AddField(FieldOperation):
    field: Field[Any, Any] = ...
    preserve_default: bool = ...
    def __init__(
        self,
        model_name: str,
        name: str,
        field: Field[Any, Any],
        preserve_default: bool = ...,
    ) -> None: ...

I need to create a similar function for collected data that can apply/revert changes based on changes made to the model the data is based on. This function would be used inside migrations in a similar way to AddField, and needs to detect whether the user is applying or reverting a migration.

Answer 1

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.