✅(backend) fix migration test using model factory

Migration tests should not import and use factories or models
directly from the code because they would not be in sync with
the database in the state that each state needs to test it.

Instead the migrator object passed as argument allows us to
retrieve a minimal version of the models in sync with the state
of the database that we are testing. What we get is a minimal
model and we need to simulate all the methods that we could have
on the real model and that are needed for testing.

Author: sampaccoud

src/backend/core/tests/migrations/test_0018_update_blank_title.py

@@ -0,0 +1,47 @@
+import pytest
+
+from core import models
+
+
+@pytest.mark.django_db
+def test_update_blank_title_migration(migrator):
+    """
+    Test that the migration fixes the titles of documents that are
+    "Untitled document", "Unbenanntes Dokument" or "Document sans titre"
+    """
+    old_state = migrator.apply_initial_migration(
+        ("core", "0017_add_fields_for_soft_delete")
+    )
+    OldDocument = old_state.apps.get_model("core", "Document")
+
+    old_english_doc = OldDocument.objects.create(
+        title="Untitled document", depth=1, path="0000001"
+    )
+    old_german_doc = OldDocument.objects.create(
+        title="Unbenanntes Dokument", depth=1, path="0000002"
+    )
+    old_french_doc = OldDocument.objects.create(
+        title="Document sans titre", depth=1, path="0000003"
+    )
+    old_other_doc = OldDocument.objects.create(
+        title="My document", depth=1, path="0000004"
+    )
+
+    assert old_english_doc.title == "Untitled document"
+    assert old_german_doc.title == "Unbenanntes Dokument"
+    assert old_french_doc.title == "Document sans titre"
+    assert old_other_doc.title == "My document"
+
+    # Apply the migration
+    new_state = migrator.apply_tested_migration(("core", "0018_update_blank_title"))
+    NewDocument = new_state.apps.get_model("core", "Document")
+
+    new_english_doc = NewDocument.objects.get(pk=old_english_doc.pk)
+    new_german_doc = NewDocument.objects.get(pk=old_german_doc.pk)
+    new_french_doc = NewDocument.objects.get(pk=old_french_doc.pk)
+    new_other_doc = NewDocument.objects.get(pk=old_other_doc.pk)
+
+    assert new_english_doc.title == None
+    assert new_german_doc.title == None
+    assert new_french_doc.title == None
+    assert new_other_doc.title == "My document"