Django models are not ajax serializable

You just need to add, in your .dumps call, a default=encode_myway argument to let simplejson know what to do when you pass it data whose types it does not know -- the answer to your "why" question is of course that you haven't told poor simplejson what to DO with one of your models' instances.

And of course you need to write encode_myway to provide JSON-encodable data, e.g.:

def encode_myway(obj):  if isinstance(obj, User):    return [obj.username,            obj.firstname,            obj.lastname,            obj.email]    # and/or whatever else  elif isinstance(obj, OtherModel):    return [] # whatever  elif ...  else:    raise TypeError(repr(obj) + " is not JSON serializable")

Basically, JSON knows about VERY elementary data types (strings, ints and floats, grouped into dicts and lists) -- it's YOUR responsibility as an application programmer to match everything else into/from such elementary data types, and in simplejson that's typically done through a function passed to default= at dump or dumps time.

Alternatively, you can use the json serializer that's part of Django, see the docs.