Validation errors are swallowed when skip_unchanged is enabled

[matthewhegarty]

Describe the bug

If you set skip_unchanged = True, you are saying that any rows that have not changed wrt to the persisted instance should be skipped (this is reported in the import results).

If you import a field which will fail validation (e.g. a bad date format), then this correctly raises a ValidationError, this is also reported in the import results.

However, if you set skip_unchanged=True and you have a field with a bad date format, you do not see the ValidationError in the results, instead the row is marked as skipped.

This is a bug because:

1. The ValidationError should be reported.
2. The row should be marked as invalid not skipped (the incoming data is modified wrt to persisted data).

This occurs because the validate_instance() method is only called if skip_row() returns False.

To Reproduce

1. Create a Resource with skip_unchanged=True
2. Create an instance of an existing entity (e.g. Author)
3. Import an invalid row to be updated.

Failing test case here.

Versions (please complete the following information):

• Django Import Export: 2.7.1
• Python 3.9
• Django 4.0

Expected behavior

ValidationErrors should be reported in the result. Invalid rows should not be skipped.

[matthewhegarty]

Fixed in release 3