FIX: Constraint error when inserting the same topic group twice

If that happens, update the value instead of raising an error.

app/models/topic_group.rb

@@ -41,14 +41,19 @@ class TopicGroup < ActiveRecord::Base
     query = <<~SQL
       INSERT INTO topic_groups (topic_id, group_id, last_read_post_number, created_at, updated_at)
       SELECT tag.topic_id, tag.group_id, :post_number, :now, :now
-      FROM topic_allowed_groups tag
-      INNER JOIN group_users gu ON gu.group_id = tag.group_id
-      WHERE gu.user_id = :user_id
-      AND tag.topic_id = :topic_id
+        FROM topic_allowed_groups tag
+        INNER JOIN group_users gu ON gu.group_id = tag.group_id
+        WHERE gu.user_id = :user_id
+        AND tag.topic_id = :topic_id
     SQL
 
     query += 'AND NOT(tag.group_id IN (:already_updated_groups))' unless updated_group_ids.length.zero?
 
+    query += <<~CONFLICT
+      ON CONFLICT(topic_id, group_id)
+      DO UPDATE SET last_read_post_number = :post_number, created_at = :now, updated_at = :now
+    CONFLICT
+
     DB.exec(
       query,
       user_id: user.id, topic_id: topic_id, post_number: post_number, now: DateTime.now, already_updated_groups: updated_group_ids

spec/models/topic_group_spec.rb

@@ -81,5 +81,12 @@ describe TopicGroup do
       expect(created_topic_group.last_read_post_number).to eq new_post_number
       expect(created_topic_group2.last_read_post_number).to eq topic2.highest_post_number
     end
+
+    it "will not raise an error if a topic group already exists" do
+      TopicGroup.create_topic_group(user, @topic.id, 3, [])
+      expect(TopicGroup.find_by(group: group, topic: @topic).last_read_post_number).to eq(3)
+      TopicGroup.create_topic_group(user, @topic.id, 10, [])
+      expect(TopicGroup.find_by(group: group, topic: @topic).last_read_post_number).to eq(10)
+    end
   end
 end