intercept changeset m2m changes

src/c3nav/editor/changes.py

@@ -7,7 +7,7 @@ from typing import TypeAlias, Literal, Annotated, Union, Type, Any
 from django.core import serializers
 from django.db import transaction
 from django.db.models import Model
-from django.db.models.fields.related import OneToOneField, ForeignKey
+from django.db.models.fields.related import OneToOneField, ForeignKey, ManyToManyField
 from django.utils import timezone
 from pydantic import ConfigDict, Discriminator
 from pydantic.fields import Field
@@ -88,7 +88,6 @@ ChangeSetChange = Annotated[
 class ChangeSetChanges(BaseSchema):
     prev_reprs: dict[ObjectReference, str] = {}
     prev_values: dict[ObjectReference, FieldValuesDict] = {}
-    prev_m2m: dict[ObjectReference, dict[str, list[int]]] = {}
     changes: list[ChangeSetChange] = []
 
 
@@ -167,8 +166,48 @@ class ChangesetOverlayManager:
         from pprint import pprint
         pprint(self.changes)
 
-    def handle_m2m_changed(self, sender: Type[Model], instance: Model, **kwargs):
-        pass
+    def handle_m2m_changed(self, sender: Type[Model], instance: Model, action: str, model: Type[Model],
+                           pk_set: set | None, reverse: bool, **kwargs):
+        if reverse:
+            raise NotImplementedError
+
+        if action.startswith("pre_"):
+            return self.handle_pre_change_instance(sender=instance._meta.model, instance=instance)
+
+        for field in instance._meta.get_fields():
+            if isinstance(field, ManyToManyField):
+                break
+        else:
+            raise ValueError
+
+        ref = ObjectReference.from_instance(instance)
+        pre_change_values = self.pre_change_values.pop(ref, None)
+        if pre_change_values:
+            self.changes.prev_values[ref] = pre_change_values
+
+        match(action):
+            case "post_add":
+                self.changes.changes.append(AddManyToManyChange(
+                    obj=ref,
+                    field=field.name,
+                    values=list(pk_set),
+                ))
+
+            case "post_remove":
+                self.changes.changes.append(RemoveManyToManyChange(
+                    obj=ref,
+                    field=field.name,
+                    values=list(pk_set),
+                ))
+
+            case "post_clear":
+                self.changes.changes.append(ClearManyToManyChange(
+                    obj=ref,
+                    field=field.name,
+                ))
+
+        from pprint import pprint
+        pprint(self.changes)
 
 
 def handle_pre_change_instance(sender: Type[Model], **kwargs):