id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
26064	Move migration operation reduction logic to their respective class.	Simon Charette	Simon Charette	"In order to avoid redundant operations during `makemigrations` and to support migration squashing [https://github.com/django/django/blob/8715205c5c0e49b21b5bbea35d904713ee188a71/django/db/migrations/optimizer.py a reduction algorithm] combines and elides a linear collection of operations.

In the case of `makemigrations` the algorithm is run on each generated `Migration.operations` individually and in the case of `squashmigration <app_label>` it is the ordered combination of all `Migration.operations` of the specified application.

The actual reduction algorithm [https://github.com/django/django/blob/8715205c5c0e49b21b5bbea35d904713ee188a71/django/db/migrations/optimizer.py#L34-L77 hard codes all possible optimizations] which makes it harder to unit test and extend (see #24109).

From my understanding these reduction methods were originally hardcoded in the `Optimizer` class for performance reason. However, based on benchmarking using a [https://gist.github.com/MarkusH/a95d9407a96fb8e63783 migration generation script provided by Markus], it looks like this might be a case of premature optimization as I couldn't get a noticeable slowdown on both `makemigrations` (with over 1000 operations generated in a single migration) and `squashmigrations` (with over 100 migrations with around 10 operations each).

Therefore I suggest to move operation reduction to their respective class.

Note that the `Operation.reduce` method could even be documented in the future to expose a public API for third party application shipping with custom `Operation` providers.

If it was made public this method could also be used to solve [https://groups.google.com/d/msg/django-developers/Ln1-IqysEwE/iL20WPT3EgAJ the data migration squashing problem described by Shai on the mailing list] by allowing specific reduction to be performed.

For example, given the following operation:

{{{#!python
class CreateInitialCountries(migrations.RunPython):
    def __init__(model_name, name_field):
        self.model_name = model_name
        self.name_field = name_field
        super().__init__(self.forwards)

    def forwards(self, apps, schema_editor):
        country_model = apps.get(self.model_name)
        for name in self.get_initial_countries_from_csv():
            country_model.objects.create(**{self.name_field: name})

    def reduce(self, operation, in_between):
        if (isinstance(operation, RenameModel) and
                self.model_name == operation.old_model_name):
            return [
                operation, CreateInitialCountries(
                    operation.new_model_name, self.name_field
                )
            ]
        elif (isinstance(operation, RenameField) and
              self.model_name == other.model_name and
              self.field_name == other.old_name):
            return [
                operation, CreateInitialCountries(
                    self.model_name, other.new_name
                )
            ]
        elif (isinstance(operation, DeleteModel) and
              self.model_name == operation.model_name):
            return [operation]
}}}

These list of operations would me reduced/optimized as follow:

{{{#!python
operations = [
    CreateModel(
        'Country',
        fields=[
            ('id', models.AutoField(primary_key=True)),
            ('name', models.CharField(max_length=50)),
        ]
    ),
    CreateInitialCountries('Country', 'name'),
    RenameField('Country', 'name', 'renamed_name'),
    RenameModel('Country', 'RenamedCountry'),
]
assert optimize(operations) == [
    CreateModel(
        'RenamedCountry',
        fields=[
            ('id', models.AutoField(primary_key=True)),
            ('renamed', models.CharField(max_length=50)),
        ]
    ),
    CreateInitialCountries('RenamedCountry', 'renamed'),
]

operations = [
    CreateModel(
        'Country',
        fields=[
            ('id', models.AutoField(primary_key=True)),
            ('name', models.CharField(max_length=50)),
        ]
    ),
    CreateInitialCountries('Country', 'name'),
    DeleteModel('Country'),
]
assert optimize(operations) == []
}}}"	Cleanup/optimization	closed	Migrations	dev	Normal	fixed		Shai Berger Loic Bistuer	Ready for checkin	1	0	0	0	0	0