Corrected problem with MySQL checks handler and related fields.

        from django.db import connection

        errors = super(DatabaseValidation, self).check_field(field, **kwargs)

        try:

        # Ignore any related fields.

        if getattr(field, 'rel', None) is None:

            field_type = field.db_type(connection)

        except AttributeError:

            # If the field is a relative field and the target model is

            # missing, then field.rel.to is not a model and doesn't have

            # `_meta` attribute.

            field_type = ''

        if (field_type.startswith('varchar') and field.unique

            if (field_type.startswith('varchar')  # Look for CharFields...

                    and field.unique  # ... that are unique

                    and (field.max_length is None or int(field.max_length) > 255)):

                errors.append(

                    checks.Error(