Add checks to ensure LockableMixin is applied before RevisionMixin

wagtail/models/__init__.py

@@ -801,6 +801,32 @@ class LockableMixin(models.Model):
     class Meta:
         abstract = True
 
+    @classmethod
+    def check(cls, **kwargs):
+        return [
+            *super().check(**kwargs),
+            *cls._check_revision_mixin(),
+        ]
+
+    @classmethod
+    def _check_revision_mixin(cls):
+        mro = cls.mro()
+        error = checks.Error(
+            "LockableMixin must be applied before RevisionMixin.",
+            hint="Move LockableMixin in the model's base classes before RevisionMixin.",
+            obj=cls,
+            id="wagtailcore.E005",
+        )
+
+        try:
+            if mro.index(RevisionMixin) < mro.index(LockableMixin):
+                return [error]
+        except ValueError:
+            # LockableMixin can be used without RevisionMixin.
+            return []
+
+        return []
+
     def with_content_json(self, content):
         """
         Similar to :meth:`RevisionMixin.with_content_json`,

wagtail/tests/test_lockable_model.py

@@ -0,0 +1,54 @@
+from django.apps import apps
+from django.core import checks
+from django.db import models
+from django.test import TestCase
+
+from wagtail.models import LockableMixin, RevisionMixin
+
+
+class TestLockableMixin(TestCase):
+    def tearDown(self):
+        # Unregister the models from the overall model registry
+        # so that it doesn't break tests elsewhere.
+        # We can probably replace this with Django's @isolate_apps decorator.
+        for package in ("wagtailcore", "wagtail.tests"):
+            try:
+                for model in (
+                    "lockablewithoutrevisionmodel",
+                    "lockableincorrectrevisionmodel",
+                    "lockablewithrevisionmodel",
+                ):
+                    del apps.all_models[package][model]
+            except KeyError:
+                pass
+        apps.clear_cache()
+
+    def test_lockable_mixin_only(self):
+        class LockableWithoutRevisionModel(LockableMixin, models.Model):
+            pass
+
+        self.assertEqual(LockableWithoutRevisionModel.check(), [])
+
+    def test_incorrect_revision_mixin_order(self):
+        class LockableIncorrectRevisionModel(
+            RevisionMixin, LockableMixin, models.Model
+        ):
+            pass
+
+        self.assertEqual(
+            LockableIncorrectRevisionModel.check(),
+            [
+                checks.Error(
+                    "LockableMixin must be applied before RevisionMixin.",
+                    hint="Move LockableMixin in the model's base classes before RevisionMixin.",
+                    obj=LockableIncorrectRevisionModel,
+                    id="wagtailcore.E005",
+                )
+            ],
+        )
+
+    def test_correct_revision_mixin_order(self):
+        class LockableWithRevisionModel(LockableMixin, RevisionMixin, models.Model):
+            pass
+
+        self.assertEqual(LockableWithRevisionModel.check(), [])