Following #23556: manage multi-apps dependencies when squashing

[Stephane "Twidi" Angel]

Following ticket #23556 and PR #3280, I found a problem with squashing on different apps, each one depending on the other at different step

app1:

• 1_auto
• 2_auto
• 3_auto, with dependency on ('app2', '2_auto')
• 4_auto

With this squash:

• 2_squashed_3 (squash of 2_auto and 3_auto)

And app2:

• 1_auto, with dependency on ('app1', '1_auto')
• 2_auto

With this squash:

• 1_squashed_2 (squash of 1_auto and 2_auto)

With this configuration, there is an error in build_graph:

File "django/db/migrations/loader.py", line 224, in build_graph
    normal[child_key].dependencies.remove(replaced)
KeyError: ('app1', '3_auto')

The reason is simple, ('app1', '3_auto') where replaced by ('app1', '2_squashed_3')

So I came up with this patch:

django/db/migrations/loader.py

@@ -219 +219 @@
                 for child_key in reverse_dependencies.get(replaced, set()):
                     if child_key in migration.replaces:
                         continue
-                    normal[child_key].dependencies.remove(replaced)
-                    normal[child_key].dependencies.append(key)
+                    # child_key may appear in a replacement
+                    if child_key in reverse_replacements:
+                        for replaced_child_key in reverse_replacements[child_key]:
+                            if replaced in replacing[replaced_child_key].dependencies:
+                                replacing[replaced_child_key].dependencies.remove(replaced)
+                                replacing[replaced_child_key].dependencies.append(key)
+                    else:
+                        normal[child_key].dependencies.remove(replaced)
+                        normal[child_key].dependencies.append(key)
             normal[key] = migration
             # Mark the replacement as applied if all its replaced ones are
             if all(applied_statuses):

Note that I loop on reverse_replacements[child_key], but I'm not sure how we can have many entries here

I'll provide a PR with two commits:

• one with a test showing the failure
• one with the patch

[Stephane "Twidi" Angel]

My Pull Request: ​https://github.com/django/django/pull/3446/