Reload doesn't check if attribute is writable

Hello!

I noticed a weird behavior and managed to isolate the problem.

I think this situation is not as uncommon as it might appear:

from mongoengine import Document, IntField, connect

__author__ = 'gabriele'

connect('test_mongoengine')


class MyModel(Document):
    meta = {
        'strict': False
    }

    int_field = IntField()
    deleted_field = IntField()


new_model = MyModel(int_field=3, deleted_field=4)
new_model.save()


class MyModel(Document):
    int_field = IntField()

    meta = {
        'strict': False
    }

    @property
    def deleted_field(self):
        return 'default'


obj = MyModel.objects(int_field=3).first()

obj.reload()

This will cause mongoengine to raise an AttributeError :

[...]/mongoengine/base/document.py", line 187, in __setattr__
    super(BaseDocument, self).__setattr__(name, value)
AttributeError: can't set attribute

This is an inconsistent behavior because the error is NOT raised when the object is first loaded, but when it is reloaded.

Start with the reproducer in the issue, then inspect mongoengine/base/document.py around setattr and the reload path. Compare reload behavior with initial loading and determine the intended handling for the read-only property; done means the inconsistent AttributeError behavior is resolved and covered by a regression test.