FIX: cannot remove tags from a topic

lib/post_revisor.rb

@@ -84,6 +84,17 @@ class PostRevisor
     end
   end
 
+  track_topic_field(:tags_empty_array) do |tc, val|
+    if val.present? && tc.guardian.can_tag_topics?
+      prev_tags = tc.topic.tags.map(&:name)
+      if !DiscourseTagging.tag_topic_by_names(tc.topic, tc.guardian, [])
+        tc.check_result(false)
+        next
+      end
+      tc.record_change('tags', prev_tags, nil)
+    end
+  end
+
   # AVAILABLE OPTIONS:
   # - revised_at: changes the date of the revision
   # - force_new_version: bypass ninja-edit window

spec/components/post_revisor_spec.rb

@@ -415,6 +415,14 @@ describe PostRevisor do
             expect(post.topic.tags.size).to eq(0)
           end
 
+          it "can remove all tags using tags_empty_array" do
+            topic.tags = [Fabricate(:tag, name: "stuff")]
+            result = subject.revise!(Fabricate(:user), { raw: "lets totally update the body", tags_empty_array: "true" })
+            expect(result).to eq(true)
+            post.reload
+            expect(post.topic.tags.size).to eq(0)
+          end
+
           it "can't add staff-only tags" do
             SiteSetting.staff_tags = "important"
             result = subject.revise!(Fabricate(:user), { raw: "lets totally update the body", tags: ['important', 'stuff'] })