no_dereference context manager is not working

[bagerard]

The no_dereference context manager suffers from the same issue as the no_subclasses context (#1865) regarding its _exit_ https://github.com/MongoEngine/mongoengine/blob/master/mongoengine/context_managers.py#L130, which means that any exceptions raised during the context gets silenced...

This means that the context is also swallowing the AssertionError being actually raised from the tests...
The following piece of code is not working since at least 0.10.0:

In [1]: import mongoengine as me
   ...: me.connect()
   ...: 
   ...: class Ref(me.Document):
   ...:     whatever = me.StringField()
   ...:     
   ...: class Doc(me.Document):
   ...:     ref = me.ReferenceField(Ref)
   ...: 
   ...: ref = Ref(whatever='').save()
   ...: Doc(ref=ref).save()
   ...: 
   ...: with me.context_managers.no_dereference(Doc):
   ...:     print Doc.objects().first().ref			# should be a DBRef but is a 'Ref object...'
Ref object

In [2]: Doc.objects().no_dereference().first().ref
Out[2]: DBRef('ref', ObjectId('5b8b07714ec5dc0d752d504b'))	# fine

I'll push a PR shortly

[bagerard]

Just like for no_subclasses, it is confusing that the context manager returns something in the _enter_ (i.e: allowing for "with ... as ..." instead of just "with ...") https://github.com/MongoEngine/mongoengine/blob/master/tests/test_context_managers.py#L86
But in the following

with no_dereference(Group) as NoDeRefGroup:
    Group is NoDeRefGroup # True!

I find this confusing because the user might think that he can mess around with the NoDeRefGroup class but he is actually using Group. May I remove the "as ..." construct (breaking change)?

[bagerard]

Closing this as it was already reported (#1245 - #1284). I'll refer to those ticket when fixing it