don't allow Followers from and to the same protocol

also make from and to required

models.py

@@ -488,8 +488,8 @@ class Follower(ndb.Model):
     STATUSES = ('active', 'inactive')
 
     # these are both subclasses of User
-    from_ = ndb.KeyProperty(name='from')
-    to = ndb.KeyProperty()
+    from_ = ndb.KeyProperty(name='from', required=True)
+    to = ndb.KeyProperty(required=True)
 
     follow = ndb.KeyProperty(Object)  # last follow activity
     status = ndb.StringProperty(choices=STATUSES, default='active')
@@ -502,6 +502,13 @@ class Follower(ndb.Model):
     # dest = ndb.StringProperty()
     # last_follow = JsonProperty()
 
+    def _pre_put_hook(self):
+        if self.from_.kind() == 'Fake' and self.to.kind() == 'Fake':
+            return
+
+        # we're a bridge! stick with bridging.
+        assert self.from_.kind() != self.to.kind(), f'from {self.from_} to {self.to}'
+
     def _post_put_hook(self, future):
         logger.info(f'Wrote {self}')
 

tests/test_models.py

@@ -16,6 +16,7 @@ from .testutil import Fake, TestCase
 import common
 from models import AtpNode, Follower, Object, OBJECT_EXPIRE_AGE, User
 import protocol
+from web import Web
 
 from .test_activitypub import ACTOR
 
@@ -216,6 +217,13 @@ class FollowerTest(TestCase):
         g.user = self.make_user('foo', cls=Fake)
         self.other_user = self.make_user('bar', cls=Fake)
 
+    def test_from_to_same_type_fails(self):
+        with self.assertRaises(AssertionError):
+            Follower(from_=Web.key_for('foo'), to=Web.key_for('bar')).put()
+
+        with self.assertRaises(AssertionError):
+            Follower.get_or_create(from_=Web(id='foo'), to=Web(id='bar'))
+
     def test_get_or_create(self):
         follower = Follower.get_or_create(from_=g.user, to=self.other_user)