Mongoengine runs validate also on non changed fields before saving.

class User(Resource):
    state = IntField(required=True)
    name = SmallStringField(required=True)

u = User()
u.state = 1
u.name = "Tamer"
u.save()

u1 = User.objects.only('name').first()
u1.name = "Radian"
u1.save() # Fails because state is required

Mongoengine runs validate on all fields before save, even if there they are not changed. Which cases the above problem. I suggest save should validate changed fields only in case of partial documents.

I am running the stable Mongoengine version from pip.

Reproduce the partial-document example with User.objects.only('name'), then trace the save and validation path for the User model. Confirm the expected behavior by ensuring changing and saving name does not fail because the unloaded required state field is absent.