'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.'

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

    from django.db import models
    varchar_fields = (models.CharField, models.CommaSeparatedIntegerField, models.SlugField)
    if (isinstance(f, varchar_fields) and f.unique and ((f.max_length is None) or (int(f.max_length) > 255))):
        msg = '"%(name)s":    %(cls)s    cannot    have    a    "max_length"    greater    than    255    when    using    "unique=True".'
        errors.add(opts, (msg % {'name': f.name, 'cls': f.__class__.__name__}))
