Skip ExclusionConstraint on non-PostgreSQL databases.

[hottwaj]

Exclusion constraints are only supported by postgres, and using them with e.g. a sqlite backend causes an exception during migrations when django attempts to insert them into the DB.

Could exclusion constraints be skipped when migrations are run for non-postgres backends instead? Perhaps with a warning that they are being skipped?

This would help in situations where multiple backends are used for testing/prototyping purposes

[Mariusz Felisiak]

Thanks for this report. We strongly recommend that you use the same database for testing and a real-life apps. There are many differences and you may encounter unexpected issues by using a different backend for testing. Therefore, we don't want to encourage users to do this by automatically skipping PostgreSQL-specific features. If you really have to do this you can skip PostgreSQL constraints/indexes in migration files, e.g.

class Migration(migrations.Migration):

    dependencies = [
        ('test_33652', '0001_initial'),
    ]

    if connection.vendor == 'postgresql':
        operations = [
            migrations.AddConstraint(
                model_name='hotelreservation',
                constraint=django.contrib.postgres.constraints.ExclusionConstraint(condition=models.Q(('cancelled', False)), expressions=[('room', '=')], name='exclude_overlapping_reservations'),
            ),
        ]
    else:
        operations = []