Expressions can only be used to update, not to insert

Question

i got this issue when trying to create object using Factory boy and unittest.mock for mocking payment

self = <django.db.models.sql.compiler.SQLInsertCompiler object at 0x7f5aa2913400>, field = <django.db.models.fields.CharField: card_id>
value = <MagicMock name='call_tap_api().get().resolve_expression()' id='140027256369728'>

    def prepare_value(self, field, value):
        """
        Prepare a value to be used in a query by resolving it if it is an
        expression and otherwise calling the field's get_db_prep_save().
        """
        if hasattr(value, 'resolve_expression'):
            value = value.resolve_expression(self.query, allow_joins=False, for_save=True)
            # Don't allow values containing Col expressions. They refer to
            # existing columns on a row, but in the case of insert the row
            # doesn't exist yet.
            if value.contains_column_references:
                raise ValueError(
                    'Failed to insert expression "%s" on %s. F() expressions '
>                   'can only be used to update, not to insert.' % (value, field)
                )
E               ValueError: Failed to insert expression "<MagicMock name='call_tap_api().get().resolve_expression()' id='140027256369728'>" on tap.TapSubscription.card_id. F() expressions can only be used to update, not to insert.

/usr/local/lib/python3.6/site-packages/django/db/models/sql/compiler.py:1171: ValueError

Hey! Without the code of the model, the factory, and the mock, it's pretty hard to get an idea of what's happening ;) If you're able to provide parts of this — maybe the part where that MagicMock is used — it would help a lot in providing a proper asnswer! — Xelnor, Jul 03 '20 at 21:48

score 0 · Accepted Answer · answered Jul 04 '20 at 07:23

this is the code that i was using and caused the issue

@pytest.mark.django_db
@tap_vcr.use_cassette(match_on=('method', 'path'))
@override_settings(TAP_SECRET_KEY='test')
@mock.patch('core.payment.gateway.tap.utils.initiate_payment')
def test_subscription_handler(client, user):

    user = UserFactory.create()
    with mock.patch('core.payment.gateway.tap.utils.call_tap_api') as call_tap_api:
        customer_id = create_customer(user)
        card_token = tockenize_card('123456789',
                                    '01',
                                    '01',
                                    '100',
                                    user.username)

        card_id = save_card(customer_id, card_token)
        instance = TapSubscriptionFactory.create(user=user, card_id= card_id)
        assert instance.id

create_customer() and tockenize_card() and save_card() are using call_tap_api() function that used to call payment api , with using @mock.patch the value i got was like <MagicMock name='call_tap_api().get().resolve_expression()' id='140651554367360'> i've solved the issue by :

@pytest.mark.django_db
@tap_vcr.use_cassette(match_on=('method', 'path'))
@override_settings(TAP_SECRET_KEY='test')
@mock.patch('core.payment.gateway.tap.utils.initiate_payment')
def test_subscription_handler(client, user):

    user = UserFactory.create()
    with mock.patch('core.payment.gateway.tap.utils.call_tap_api') as call_tap_api:
        customer_id = user.tap_customer_id
        card_token = factory.fuzzy.FuzzyInteger(1, 9999)

        card_id = factory.fuzzy.FuzzyInteger(1, 9999)
        instance = TapSubscriptionFactory.create(user=user, card_id= card_id)
        assert instance.id

now customer_id and card_token and card_id get a generated value instead of

<MagicMock name='call_tap_api().get().resolve_expression()' id='140651554367360'>

score 0 · Answer 2 · answered Jan 25 '23 at 10:30

I was getting a similar error while trying to mock a Celery task delay function:

@patch('some_app.tasks.some_task.delay')
    def test_create_graph_report_mutation(self, mock_task):
        # Some code that triggers the task indirectly.
        mock_task.assert_called()

To fix this, I also mocked the function which was actually triggering the task.

@patch('some_app.models.some_function')
@patch('some_app.tasks.some_task.delay')
    def test_create_graph_report_mutation(self, mock_task, mock_some_function):
        # Some code that triggers the task indirectly.
        mock_some_function.assert_called()
        mock_task.assert_called()

Conclusion: This leads me to believe, and the solution that has been given, that if we mock a nested function and the testing code indirectly depends on it, we may either eliminate this reliance or mock the parent function as well.

Expressions can only be used to update, not to insert

2 Answers2