Added django style choices

mongoengine/base.py

@@ -100,9 +100,9 @@ class BaseField(object):
     def _validate(self, value):
         # check choices
         if self.choices is not None:
-            if value not in self.choices:
-                raise ValidationError("Value must be one of %s."
-                    % unicode(self.choices))
+            option_keys = [option_key for option_key, option_value in self.choices]
+            if value not in option_keys:
+                raise ValidationError("Value must be one of %s." % unicode(option_keys))
 
         # check validation argument
         if self.validation is not None:

mongoengine/fields.py

@@ -12,7 +12,6 @@ import datetime
 import decimal
 import gridfs
 import warnings
-import types
 
 
 __all__ = ['StringField', 'IntField', 'FloatField', 'BooleanField',