API: add endpoint to show by what objects an object is used (#995)

* core: add used_by API to show what objects are affected before deletion

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* web/elements: add support for used_by API

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* core: add authentik_used_by_shadows to shadow other models

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* web: implement used_by API

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* *: fix duplicate imports

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* core: add action field to used_by api

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* web: add UI for used_by action

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* web: add notice to tenant form

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* core: fix naming in used_by

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* web: check length for used_by

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* core: fix used_by for non-pk models

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* *: improve __str__ on models

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* core: add support for many to many in used_by

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

authentik/core/api/used_by.py

@@ -0,0 +1,102 @@
+"""used_by mixin"""
+from enum import Enum
+from inspect import getmembers
+
+from django.db.models.base import Model
+from django.db.models.deletion import SET_DEFAULT, SET_NULL
+from django.db.models.manager import Manager
+from drf_spectacular.utils import extend_schema
+from guardian.shortcuts import get_objects_for_user
+from rest_framework.decorators import action
+from rest_framework.fields import CharField, ChoiceField
+from rest_framework.request import Request
+from rest_framework.response import Response
+
+from authentik.core.api.utils import PassiveSerializer
+
+
+class DeleteAction(Enum):
+    """Which action a delete will have on a used object"""
+
+    CASCADE = "cascade"
+    CASCADE_MANY = "cascade_many"
+    SET_NULL = "set_null"
+    SET_DEFAULT = "set_default"
+
+
+class UsedBySerializer(PassiveSerializer):
+    """A list of all objects referencing the queried object"""
+
+    app = CharField()
+    model_name = CharField()
+    pk = CharField()
+    name = CharField()
+    action = ChoiceField(choices=[(x.name, x.name) for x in DeleteAction])
+
+
+def get_delete_action(manager: Manager) -> str:
+    """Get the delete action from the Foreign key, falls back to cascade"""
+    if hasattr(manager, "field"):
+        if manager.field.remote_field.on_delete.__name__ == SET_NULL.__name__:
+            return DeleteAction.SET_NULL.name
+        if manager.field.remote_field.on_delete.__name__ == SET_DEFAULT.__name__:
+            return DeleteAction.SET_DEFAULT.name
+    if hasattr(manager, "source_field"):
+        return DeleteAction.CASCADE_MANY.name
+    return DeleteAction.CASCADE.name
+
+
+class UsedByMixin:
+    """Mixin to add a used_by endpoint to return a list of all objects using this object"""
+
+    @extend_schema(
+        responses={200: UsedBySerializer(many=True)},
+    )
+    @action(detail=True, pagination_class=None, filter_backends=[])
+    # pylint: disable=invalid-name, unused-argument, too-many-locals
+    def used_by(self, request: Request, *args, **kwargs) -> Response:
+        """Get a list of all objects that use this object"""
+        # pyright: reportGeneralTypeIssues=false
+        model: Model = self.get_object()
+        used_by = []
+        shadows = []
+        for attr_name, manager in getmembers(model, lambda x: isinstance(x, Manager)):
+            if attr_name == "objects":  # pragma: no cover
+                continue
+            manager: Manager
+            if manager.model._meta.abstract:
+                continue
+            app = manager.model._meta.app_label
+            model_name = manager.model._meta.model_name
+            delete_action = get_delete_action(manager)
+
+            # To make sure we only apply shadows when there are any objects,
+            # but so we only apply them once, have a simple flag for the first object
+            first_object = True
+
+            for obj in get_objects_for_user(
+                request.user, f"{app}.view_{model_name}", manager
+            ).all():
+                # Only merge shadows on first object
+                if first_object:
+                    shadows += getattr(
+                        manager.model._meta, "authentik_used_by_shadows", []
+                    )
+                first_object = False
+                serializer = UsedBySerializer(
+                    data={
+                        "app": app,
+                        "model_name": model_name,
+                        "pk": str(obj.pk),
+                        "name": str(obj),
+                        "action": delete_action,
+                    }
+                )
+                serializer.is_valid()
+                used_by.append(serializer.data)
+        # Check the shadows map and remove anything that should be shadowed
+        for idx, user in enumerate(used_by):
+            full_model_name = f"{user['app']}.{user['model_name']}"
+            if full_model_name in shadows:
+                del used_by[idx]
+        return Response(used_by)