how to make south aware of custom field

south_custom_field.py

"""
Following error will occur if there's custom field. 

```
 ! Cannot freeze field 'myapp.mymodel.entity_type'
 ! (this field has class myapp.fields.EntityTypeField)

 ! South cannot introspect some fields; this is probably because they are custom
 ! fields. If they worked in 0.6 or below, this is because we have removed the
 ! models parser (it often broke things).
 ! To fix this, read http://south.aeracode.org/wiki/MyFieldsDontWork
```

Simply add the following lines to make south aware of the custom field.
"""

from south.modelsinspector import add_introspection_rules
add_introspection_rules([], ["^myapp\.fields\.EntityTypeField"])

class EntityTypeField(models.Field):
    def __init__(self, max_length=10, *args, **kwargs):
        kwargs['max_length'] = max_length
        super(EntityTypeField, self).__init__(*args, **kwargs)

    def db_type(self, connection):
        return 'char(%s)' % self.max_length