How can I set a table constraint “deferrable initially deferred” in django model?

Question

I am trying to set a constraint to a table model in django with a postgresql database.

I can do it via postgresql with this sentence:

ALTER TABLE pub         


        

Answer 1

I would do this via a single migration. First programatically get the unique constraint name, then drop and re-add (since altering it seems to only work for FK constraints, not unique constraints). Add reverse migration that undoes this too.

from django.db import migrations, connection



def _make_deferrable(apps, schema_editor):
    """
    Change the unique constraint to be deferrable
    """
    # Get the db name of the constraint
    MyModel = apps.get_model('myapp', 'MyModel')
    CONSTRAINT_NAME = schema_editor._constraint_names(MYModel,
                                                                   ['col1', 'col2'],
                                                                   unique=True)[0]
    TABLE_NAME = MyModel._meta.db_table


    # Drop then re-add with deferrable as ALTER doesnt seem to work for unique constraints in psql
    with schema_editor.connection.create_cursor() as curs:
        curs.execute(
            f'ALTER TABLE {TABLE_NAME} DROP CONSTRAINT "{CONSTRAINT_NAME}";'
        )
        curs.execute(
            f'ALTER TABLE {TABLE_NAME} ADD CONSTRAINT'
            f' {CONSTRAINT_NAME}'
            f' UNIQUE (col1, col2) DEFERRABLE INITIALLY DEFERRED;'
        )


def _unmake_deferrable(apps, schema_editor):
    """
    Reverse the unique constraint to be not deferrable
    """
    # Get the db name of unique constraint
    MyModel = apps.get_model('myapp', 'MyModel')
    CONSTRAINT_NAME = schema_editor._constraint_names(MyModel,
                                                                   ['col1', 'col2'],
                                                                   unique=True)[0]
    TABLE_NAME = MyModel._meta.db_table

    with schema_editor.connection.create_cursor() as curs:
        curs.execute(
            f'ALTER TABLE {TABLE_NAME} DROP CONSTRAINT "{CONSTRAINT_NAME}";'
        ) 
        curs.execute(
            f'ALTER TABLE {TABLE_NAME} ADD CONSTRAINT'
            f' {CONSTRAINT_NAME}'
            f' UNIQUE (col1, col2) NOT DEFERRABLE;'
        )

class Migration(migrations.Migration):

    dependencies = [
        ('myapp', ''),
    ]

    operations = [
        migrations.RunPython(code=_make_deferrable,  reverse_code=_unmake_deferrable)
    ]