from django.db.backends import BaseDatabaseValidation

class DatabaseValidation(BaseDatabaseValidation):

    def validate_field(self, errors, opts, f):

        """

        There are some field length restrictions for MySQL:

        - Prior to version 5.0.3, character fields could not exceed 255

          characters in length.

        - No character (varchar) fields can have a length exceeding 255

          characters if they have a unique index on them.

        """

        from django.db import models

        from django.db import connection

        db_version = connection.get_server_version()

        varchar_fields = (models.CharField, models.CommaSeparatedIntegerField,

                models.SlugField)

        if isinstance(f, varchar_fields) and f.max_length > 255:

            if db_version < (5, 0, 3):

                msg = '"%(name)s": %(cls)s cannot have a "max_length" greater than 255 when you are using a version of MySQL prior to 5.0.3 (you are using %(version)s).'

            elif f.unique == True:

                msg = '"%(name)s": %(cls)s cannot have a "max_length" greater than 255 when using "unique=True".'

            else:

                msg = None

            if msg:

                errors.add(opts, msg % {'name': f.name, 'cls': f.__class__.__name__, 'version': '.'.join([str(n) for n in db_version[:3]])})