Multiple fields with the same db_field now raises Exception

Closes #329

AUTHORS

@@ -85,3 +85,4 @@ that much better:
  * Alexander G. Morano
  * jwilder
  * Joe Shaw
+ * Adam Flynn

docs/changelog.rst

@@ -5,6 +5,7 @@ Changelog
 Changes in dev
 ==============
 
+- Added Now raises an InvalidDocumentError when declaring multiple fields with the same db_field
 - Added InvalidQueryError when calling with_id with a filter
 - Added support for DBRefs in distinct()
 - Fixed issue saving False booleans

mongoengine/base.py

@@ -492,6 +492,7 @@ class DocumentMetaclass(type):
         attrs['_superclasses'] = superclasses
 
         # Add the document's fields to the _fields attribute
+        field_names = {}
         for attr_name, attr_value in attrs.items():
             if hasattr(attr_value, "__class__") and \
                issubclass(attr_value.__class__, BaseField):
@@ -499,8 +500,13 @@ class DocumentMetaclass(type):
                 if not attr_value.db_field:
                     attr_value.db_field = attr_name
                 doc_fields[attr_name] = attr_value
+                field_names[attr_value.db_field] = field_names.get(attr_value.db_field, 0) + 1
+
+        duplicate_db_fields = [k for k, v in field_names.items() if v > 1]
+        if duplicate_db_fields:
+            raise InvalidDocumentError("Multiple db_fields defined for: %s " % ", ".join(duplicate_db_fields))
         attrs['_fields'] = doc_fields
-        attrs['_db_field_map'] = dict([(k, v.db_field) for k, v in doc_fields.items() if k!=v.db_field])
+        attrs['_db_field_map'] = dict([(k, v.db_field) for k, v in doc_fields.items() if k != v.db_field])
         attrs['_reverse_db_field_map'] = dict([(v, k) for k, v in attrs['_db_field_map'].items()])
 
         from mongoengine import Document, EmbeddedDocument, DictField

tests/document.py

@@ -2182,6 +2182,16 @@ class DocumentTest(unittest.TestCase):
 
         BlogPost.drop_collection()
 
+    def test_duplicate_db_fields_raise_invalid_document_error(self):
+        """Ensure a InvalidDocumentError is thrown if duplicate fields
+        declare the same db_field"""
+
+        def throw_invalid_document_error():
+            class Foo(Document):
+                name = StringField()
+                name2 = StringField(db_field='name')
+
+        self.assertRaises(InvalidDocumentError, throw_invalid_document_error)
 
     def test_reverse_delete_rule_cascade_and_nullify(self):
         """Ensure that a referenced document is also deleted upon deletion.