`choices` not validated after update

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Hi everyone, I was trying to figure out how to properly use choices in a given field when defining a schema and I'm not sure if I've stumbled into something or if I'm misunderstanding the intended behaviour.

Say I have something as simple as this:

class MockJob(Document):
    job_status = StringField(required=True, choices=['foo', 'bar'])

If I now create and save a valid document:

mock_document = MockJob(job_status='foo')
mock_document.save(validate=True)

all goes as expected, validation succeeds because 'foo' is one of the admissible choices and I can check in the database that the document is there with the job_status field populated.

Now the part that puzzles me. If I update the field to an invalid value:

mock_document.update(job_status='whatever')

then the update succeeds without any validation errors, and I can see in the database that the document has indeed changed to have an invalid status.

So my first question would be: I've noticed that when updating the document there is a call to the validate method of the field's subclass (in the example StringField.validate) but I'm wondering if it would be preferable to call BaseField._validate instead? It seems that this way it would call BaseField._validate_choices along the way?

But this leads to the second question. When manually enforcing this validation:

mock_document.validate()

no errors are raised either, and the reason is that mock_document._data.get('job_status') is still 'foo'. How come there is a mismatch in this field between the value as stored in mock_document and the value as stored in the database?

I hope the example makes sense, I've kept it as bare-bones as possible. Any help with this would be greatly appreciated!

PS: I am using mongoengine v0.27.0.