FIX: delete synonym tags if other synonyms are already exist. (#21885)

When a topic already has multiple synonym tags of a target tag, if we try to update the "`tag_id`" column to target tag id then it will raise a unique violation error since there are multiple synonyms present in the topic. So before doing that action, we must delete the problematic tags so the topic has only one synonym tag to update.

This is not an issue when the topic has a target tag already along with synonyms.

lib/discourse_tagging.rb

@@ -773,6 +773,16 @@ module DiscourseTagging
     successful = existing.select { |t| !t.errors.present? }
     synonyms_ids = successful.map(&:id)
     TopicTag.where(topic_id: target_tag.topics.with_deleted, tag_id: synonyms_ids).delete_all
+    TopicTag.joins(DB.sql_fragment(<<~SQL, synonyms_ids: synonyms_ids)).delete_all
+      INNER JOIN (
+        SELECT MIN(id) AS id, topic_id
+          FROM topic_tags
+          WHERE tag_id IN (:synonyms_ids)
+          GROUP BY topic_id
+      ) AS tt ON tt.id < topic_tags.id
+                  AND tt.topic_id = topic_tags.topic_id
+                  AND topic_tags.tag_id IN (:synonyms_ids)
+    SQL
     TopicTag.where(tag_id: synonyms_ids).update_all(tag_id: target_tag.id)
     Scheduler::Defer.later "Update tag topic counts" do
       Tag.ensure_consistency!

spec/lib/discourse_tagging_spec.rb

@@ -1552,6 +1552,19 @@ RSpec.describe DiscourseTagging do
       expect(tag2.reload.target_tag).to eq(tag1)
     end
 
+    it "replaces topic synonym tags with target tag" do
+      tag4, tag5 = 2.times.collect { Fabricate(:tag) }
+      topic = Fabricate(:topic, tags: [tag2, tag3, tag4])
+      expect {
+        expect(DiscourseTagging.add_or_create_synonyms_by_name(tag1, [tag2.name, tag3.name])).to eq(
+          true,
+        )
+      }.to_not change { Tag.count }
+      expect_same_tag_names(tag1.reload.synonyms, [tag2, tag3])
+      expect_same_tag_names(topic.reload.tags, [tag1, tag4])
+      expect(tag5.reload).to be_present
+    end
+
     it "can create new tags" do
       expect {
         expect(DiscourseTagging.add_or_create_synonyms_by_name(tag1, ["synonym1"])).to eq(true)