should it assign default value for fields in __only_fields but without data in mongodb?

In [19]: class Foo(models.Document):
   ....:     foo = models.ListField()
   ....:     

In [20]: Foo.objects.delete()
Out[20]: 0

In [21]: foo = Foo.objects.create()

In [22]: foo.foo
Out[22]: []

In [23]: foo.update(unset__foo=1)
Out[23]: 1

In [24]: foo = Foo.objects.only('foo').first()

In [25]: foo.foo

In [26]: 

there is an issue fixed by https://github.com/MongoEngine/mongoengine/issues/399

but you can see line#23 will remove foo from mongodb
then i want to get Foo with foo field only, now foo will be in __only_fields and skip assign default value in https://github.com/MongoEngine/mongoengine/blob/v0.10.0/mongoengine/base/document.py#L93
but values have no foo value either https://github.com/MongoEngine/mongoengine/blob/v0.10.0/mongoengine/base/document.py#L111
so it have not assign any value for foo

i don't know the rule how to handle this situation, but it is a little annoying that i have to check foo.foo before i can use it as usual

Reproduce the __only_fields case from the issue, then inspect mongoengine/base/document.py around lines 93 and 111 to trace default assignment when the selected field is absent from MongoDB. Check existing tests for partial document loading and define the expected behavior for Foo.objects.only('foo').first(); done means the missing ListField is handled consistently without requiring a pre-access check.