Continue to write "subscriber" SRMember relation objects.

These are being used by the data team and will need to be written
until they are able to read the new `SubscribedSubredditsByAccount`
from Cassandra.

r2/r2/models/subreddit.py

@@ -57,6 +57,7 @@
 from r2.lib.strings import strings, Score
 from r2.lib.filters import _force_unicode
 from r2.lib.db import tdb_cassandra
+from r2.lib.db.tdb_sql import CreationError
 from r2.models.wiki import WikiPage, ImagesByWikiPage
 from r2.models.trylater import TryLater, TryLaterBySubject
 from r2.lib.merge import ConflictException
@@ -1135,17 +1136,20 @@ def is_subscriber(self, user):
     def add_subscriber(self, user):
         SubscribedSubredditsByAccount.create(user, self)
         SubscriptionsByDay.create(self, user)
+        add_legacy_subscriber(self, user)
         self._incr('_ups', 1)
 
     @classmethod
     def subscribe_multiple(cls, user, srs):
         SubscribedSubredditsByAccount.create(user, srs)
         SubscriptionsByDay.create(srs, user)
         for sr in srs:
+            add_legacy_subscriber(sr, user)
             sr._incr('_ups', 1)
 
     def remove_subscriber(self, user):
         SubscribedSubredditsByAccount.destroy(user, self)
+        remove_legacy_subscriber(self, user)
         self._incr('_ups', -1)
 
     @classmethod
@@ -2436,6 +2440,21 @@ def is_superuser(self):
 )
 
 
+def add_legacy_subscriber(sr, user):
+    rel = SRMember(sr, user, "subscriber")
+    try:
+        rel._commit()
+    except CreationError:
+        pass
+
+
+def remove_legacy_subscriber(sr, user):
+    rels = SRMember._fast_query([sr], [user], "subscriber")
+    rel = rels.get((sr, user, "subscriber"))
+    if rel:
+        rel._delete()
+
+
 class SubredditTempBan(object):
     def __init__(self, sr, kind, victim, banner, duration):
         self.sr = sr._id36