Migrations using MySQL fail for constraints using composite indexes

[geertjanvdk]

When running migrations which create (unique) composite indexes, the MySQL introspection fails to get the correct index:

ValueError: Found wrong number (0) of constraints for product_versions(product_id, ordering)

This was previously reported as fixed (see https://code.djangoproject.com/ticket/28697), but it is not. Using 1.11.5, and 1.11.9, it fails.

The fix is very easy: in DatabaseIntrospection.get_constraints(), an ORDER BY is missing when selecting from the table INFOMRATION_SCHEMA.KEY_COLUMN_USAGE:

Thus:

    def get_constraints(self, cursor, table_name):
        """
        Retrieves any constraints or keys (unique, pk, fk, check, index) across one or more columns.
        """
        constraints = {}
        # Get the actual constraint names and columns
        name_query = """
            SELECT kc.`constraint_name`, kc.`column_name`,
                kc.`referenced_table_name`, kc.`referenced_column_name`
            FROM information_schema.key_column_usage AS kc
            WHERE
                kc.table_schema = DATABASE() AND
                kc.table_name = %s
            ORDER BY kc.`constraint_name`, kc.`ordinal_position`
        """
       ....

The ORDER BY assures the order the columns were specified. the SHOW INDEX statement does this by default.

The above should work for all MySQL versions supported.

Note that Django 2.0 has the same issue.

Patch available ​https://github.com/geertjanvdk/django/commit/ab0bcb327c51aab71f58789eb55a7981dcb06fdb

[Hasan Ramezani]

Seems it has been fixed in ​https://github.com/django/django/commit/4fba321a457e52df77d0b512d043019e9f6b0bc9
I Added a patch to test it.

Question: Do we need to add kc.'constraint_name' to ORDER BY as well?

[Mariusz Felisiak]

kc.'constraint_name' in an ORDER BY should not be necessary. I think we can close this issue because ordering was added in 4fba321a457e52df77d0b512d043019e9f6b0bc9 and creating a regression test may not be feasible (see ​discussion).