FIX: Fix ActiveRecord API that was removed in Rails 5.1.

app/services/post_alerter.rb

@@ -473,7 +473,7 @@ class PostAlerter
     post.topic_links.where(reflection: false).map do |link|
       linked_post = link.link_post
       if !linked_post && topic = link.link_topic
-        linked_post = topic.posts(post_number: 1).first
+        linked_post = topic.posts.find_by(post_number: 1)
       end
       (linked_post && post.user_id != linked_post.user_id && linked_post.user) || nil
     end.compact

spec/services/post_alerter_spec.rb

@@ -257,7 +257,6 @@ describe PostAlerter do
       # don't notify on reflection
       post1.reload
       expect(PostAlerter.new.extract_linked_users(post1).length).to eq(0)
-
     end
 
     it "triggers :before_create_notifications_for_users" do
@@ -606,4 +605,29 @@ describe PostAlerter do
       end
     end
   end
+
+  describe '#extract_linked_users' do
+    let(:topic) { Fabricate(:topic) }
+    let(:post) { Fabricate(:post, topic: topic) }
+    let(:post2) { Fabricate(:post) }
+
+    describe 'when linked post has been deleted' do
+      let(:topic_link) do
+        TopicLink.create!(
+          url: "/t/#{topic.id}",
+          topic_id: topic.id,
+          link_topic_id: post2.topic.id,
+          link_post_id: nil,
+          post_id: post.id,
+          user: user,
+          domain: 'test.com'
+        )
+      end
+
+      it 'should use the first post of the topic' do
+        topic_link
+        expect(PostAlerter.new.extract_linked_users(post.reload)).to eq([post2.user])
+      end
+    end
+  end
 end