Added a possibility to define a base class for fields from a DictField (instead of using BaseField). This is important if you want to use field-based query abilities like StringField's startswith/endswith/contains. Just define `basecls´ when defining your DictField. Example:

class Test(Document):

    name = StringField()
    translations = DictField(basecls=StringField)

Without basecls defined:

> Test.objects(translations__german__startswith='Deutsch')
[]

With basecls set to StringField:

> Test.objects(translations__german__startswith='Deutsch')
[<Test: Test object>]

mongoengine/queryset.py

@@ -313,7 +313,7 @@ class QuerySet(object):
             op = None
             if parts[-1] in operators + match_operators:
                 op = parts.pop()
-            
+
             if _doc_cls:
                 # Switch field names to proper names [set in Field(name='foo')]
                 fields = QuerySet._lookup_field(_doc_cls, parts)