Field accessability in DynamicDocuments

A DynamicDocument take arbitrary named fields, but there is a problem with the mongoengine API:

class Test(DynamicDocument):
    name = StringField()

t = Test()

Let's say I would like a dynamic field called "validate", this is what happens:

Try number one:

t['validate'] = "foo"

Throws a KeyError exception, which I find odd, since I have a DynamicDocument.

Try number two:

t.validate = "foo"
t.save()

Throws a TypeError exception, because t.validate is no longer a function.

Try number three:

Test.from_json('{"validate": "foo"}').save()

Throws also a TypeError exception...

Try number four:

t.update(set__validate="foo")
t['validate']

Throws yet another KeyError exception, so the custom field data is lost.

And here my suggestion for an API change:

DynamicDocument.setitem(self, name) should be aware if it acts in a DynamicDocument and accept keys that are not part of self._fields.

BaseDocument.setattr(self, name, value) must only be used as a syntactical-sugar, since it can not always represent all fields.

The dictionary-style field access should be the preferred access style, since not all fields can be mapped as class attributes.

Document.from_json(cls, json_data) should also use the dictionary-style field access, so it can be safely applied to DynamicDocuments.

Start by reading DynamicDocument.setitem, BaseDocument.setattr, and Document.from_json, focusing on how dynamic field names are handled and where reserved names fail. Done means dictionary-style access and JSON loading preserve dynamic fields such as "validate" without raising the reported exceptions or losing the field data.