Bug: ListField may lost value if do iterate before some edit operation

[RyanKung]

In this sample, I append data to list, saved, then do the 'for' loop, but when I call remove(), the append data was missed, and the 'remove' func didn't work.

# -*- coding: utf-8 -*-

import unittest
from mongoengine import *

class A(Document):
    list_b = ListField(ReferenceField('B'))

    def add_b_to_a(self, b):
        self.list_b.append(b)
        self.save()

class B(Document):
    pass


class DBTest(unittest.TestCase):

    def setUp(self):
        connect('test_mongoengine')

    def tearDown(self):
        A.drop_collection()
        B.drop_collection()

    def test_mongoengine_with_iterate(self):
        a = A().save()
        b = B().save()
        b_1 = B().save()

        a.list_b.append(b)
        a.save()
        a.add_b_to_a(b_1)

        for b in a.list_b: pass

        a.list_b.remove(b)

        self.assertTrue(b_1 in a.list_b)
        self.assertFalse(b in a.list_b)

    def test_mongoengine_without_iterate(self):
        a = A().save()
        b = B().save()
        b_1 = B().save()

        a.list_b.append(b)
        a.save()
        a.add_b_to_a(b_1)

        a.list_b.remove(b)

        self.assertTrue(b_1 in a.list_b)
        self.assertFalse(b in a.list_b)

F.
======================================================================
FAIL: test_mongoengine_with_iterate (test.DBTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "XXXX/dev/test.py", line 39, in test_mongoengine_with_iterate
    self.assertTrue(b_1 in a.list_b)
AssertionError: False is not true

----------------------------------------------------------------------
Ran 2 tests in 0.017s

FAILED (failures=1)

[JohnAD]

I strongly suspect that this issue was related to #1318; which is now resolved. But, unfortunately, the example code above contains a likely-unintentional bug. In testing the iteration, the following line is used:

    for b in a.list_b: pass

This overwrites the original meaning of b. In fact, the for loop would leave the value of b to be b_1. So when:

    a.list_b.remove(b)

is then executed, it is b_1 that is actually removed not the original b. In fact, the original reference to b is lost, so there is no way to remove it with the remove function. By changing the iteration to not overwrite the meaning of b :

    for x in a.list_b: pass

Now the code functions as expected against the latest commit of master. I recommend closing this issue.

[wojcikstefan]

Thanks for a thorough explanation @JohnAD!

[RyanKung]

Wow, That's my mistake, thanks for pointing it out, @JohnAD