models: Extract zerver.models.user_topics.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/models/user_topics.py

@@ -0,0 +1,76 @@
+from datetime import datetime, timezone
+
+from django.db import models
+from django.db.models import CASCADE
+from django.db.models.functions import Lower, Upper
+from typing_extensions import override
+
+from zerver.models.constants import MAX_TOPIC_NAME_LENGTH
+from zerver.models.recipients import Recipient
+from zerver.models.streams import Stream
+from zerver.models.users import UserProfile
+
+
+class UserTopic(models.Model):
+    user_profile = models.ForeignKey(UserProfile, on_delete=CASCADE)
+    stream = models.ForeignKey(Stream, on_delete=CASCADE)
+    recipient = models.ForeignKey(Recipient, on_delete=CASCADE)
+    topic_name = models.CharField(max_length=MAX_TOPIC_NAME_LENGTH)
+    # The default value for last_updated is a few weeks before tracking
+    # of when topics were muted was first introduced.  It's designed
+    # to be obviously incorrect so that one can tell it's backfilled data.
+    last_updated = models.DateTimeField(default=datetime(2020, 1, 1, 0, 0, tzinfo=timezone.utc))
+
+    class VisibilityPolicy(models.IntegerChoices):
+        # A normal muted topic. No notifications and unreads hidden.
+        MUTED = 1, "Muted topic"
+
+        # This topic will behave like an unmuted topic in an unmuted stream even if it
+        # belongs to a muted stream.
+        UNMUTED = 2, "Unmuted topic in muted stream"
+
+        # This topic will behave like `UNMUTED`, plus some additional
+        # display and/or notifications priority that is TBD and likely to
+        # be configurable; see #6027. Not yet implemented.
+        FOLLOWED = 3, "Followed topic"
+
+        # Implicitly, if a UserTopic does not exist, the (user, topic)
+        # pair should have normal behavior for that (user, stream) pair.
+
+        # We use this in our code to represent the condition in the comment above.
+        INHERIT = 0, "User's default policy for the stream."
+
+    visibility_policy = models.SmallIntegerField(
+        choices=VisibilityPolicy.choices, default=VisibilityPolicy.MUTED
+    )
+
+    class Meta:
+        constraints = [
+            models.UniqueConstraint(
+                "user_profile",
+                "stream",
+                Lower("topic_name"),
+                name="usertopic_case_insensitive_topic_uniq",
+            ),
+        ]
+
+        indexes = [
+            models.Index("stream", Upper("topic_name"), name="zerver_mutedtopic_stream_topic"),
+            # This index is designed to optimize queries fetching the
+            # set of users who have special policy for a stream,
+            # e.g. for the send-message code paths.
+            models.Index(
+                fields=("stream", "topic_name", "visibility_policy", "user_profile"),
+                name="zerver_usertopic_stream_topic_user_visibility_idx",
+            ),
+            # This index is useful for handling API requests fetching the
+            # muted topics for a given user or user/stream pair.
+            models.Index(
+                fields=("user_profile", "visibility_policy", "stream", "topic_name"),
+                name="zerver_usertopic_user_visibility_idx",
+            ),
+        ]
+
+    @override
+    def __str__(self) -> str:
+        return f"({self.user_profile.email}, {self.stream.name}, {self.topic_name}, {self.last_updated})"