Avoid room queries for messages without mentions

test/models/message/mentionee_test.rb

@@ -0,0 +1,38 @@
+require "test_helper"
+
+class Message::MentioneeTest < ActiveSupport::TestCase
+  test "ordinary messages return an empty user relation without loading their room" do
+    message = rooms(:pets).messages.create!(creator: users(:jason), body: "<p>Fish &amp; chips 🌍</p>").reload
+    message.body
+    assert_not message.association(:room).loaded?
+    queries = []
+    subscriber = ->(*, payload) { queries << payload[:sql] unless payload[:name] == "SCHEMA" || payload[:cached] }
+
+    ActiveSupport::Notifications.subscribed(subscriber, "sql.active_record") do
+      mentionees = message.mentionees
+      assert_equal User, mentionees.klass
+      assert_empty mentionees.to_a
+      assert_empty mentionees.ids
+      assert_empty mentionees.active_bots.excluding(users(:jason)).to_a
+    end
+
+    assert_empty queries
+    assert_not message.association(:room).loaded?
+    assert_not message.changed?
+  end
+
+  test "signed mentions follow a moved message and fresh membership changes" do
+    message = rooms(:designers).messages.create!(creator: users(:david), body: "<p>Hey #{mention_attachment_for(:kevin)}</p>")
+    assert_equal [ users(:kevin).id ], message.reload.mentionees.ids
+    Message.where(id: message.id).update_all(room_id: rooms(:pets).id)
+    message = Message.find(message.id)
+    assert_empty message.mentionees.ids
+
+    membership = Membership.create!(room: rooms(:pets), user: users(:kevin))
+    assert_equal [ users(:kevin).id ], message.mentionees.ids
+    membership.delete
+    assert_empty message.mentionees.ids
+    assert_equal rooms(:pets).id, message.room_id
+    assert_not message.changed?
+  end
+end