from django.core import checks
from django.db.backends import BaseDatabaseValidation


class DatabaseValidation(BaseDatabaseValidation):
    def check_field(self, field, **kwargs):
        """
        MySQL has the following field length restriction:
        No character (varchar) fields can have a length exceeding 255
        characters if they have a unique index on them.
        """
        from django.db import connection

        errors = super(DatabaseValidation, self).check_field(field, **kwargs)
        try:
            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
                and (field.max_length is None or int(field.max_length) > 255)):
            errors.append(
                checks.Error(
                    ('Under mysql backend, the field cannot have a "max_length" '
                     'greated than 255 when it is unique.'),
                    hint=None,
                    obj=field,
                    id='E047',
                )
            )
        return errors