Empty DictField in ListField

When I give an empty dict (“{}”) to a DictField inside a ListField, I get this error when trying to save the object:

bson.errors.InvalidDocument: Cannot encode object: <marshmallow.missing>

A minimal example to reproduce:

... initialize db ...
from umongo import fields, Document
@instance.register
class ListTest(Document):
    dict = fields.ListField(fields.DictField())

new = ListTest(dict=[{}])
new.commit()

The problem is, that the _serialize_to_mongo of DictField replaces empty dicts with the missing object:

def _serialize_to_mongo(self, obj):
    if not obj:
        return missing
    return dict(obj)

See http://umongo.readthedocs.io/en/latest/_modules/umongo/fields.html#DictField

Why isn’t it possible to save an empty Dict? Especially in a ListField there is a difference between having an empty dict and not having an object at all.

[touilleMan]

Seems pretty reasonable to me, @lafrech feel free to make this change into the master 😉

[lafrech]

I modified _serialize_to_mongo like this:

    def _serialize_to_mongo(self, obj):
        if obj is None:
            return missing
        return dict(obj)

It only breaks the test that checks precisely the behavior I’m changing:

        # Empty dict is considered as missing field
        d2.set('dict', {})
>       assert d2.to_mongo() == {}
E       AssertionError: assert {'in_mongo_dict': {}} == {}
E         Left contains more items:
E         {'in_mongo_dict': {}}
E         Use -v to get the full diff

Serializing empty dict as {} rather than missing seems reasonable to me, not only in the “dict in the list” case.

@touilleMan, any opinion about this?