FIX: check min tag count requirement when change category (#12252)

When a category is created, we can set `minimum_required_tags` property.

When the topic is created, we are checking that field and ensuring that the minimum amount of tags were added - unless topic created by a staff member.

Problem is that validation is skipped when we change the category from for example category with 0 tags required to the category with 1 tag required.

Changing category is kind of the unicorn as it is a complicated operation:

https://github.com/discourse/discourse/blob/master/lib/post_revisor.rb#L84

https://github.com/discourse/discourse/blob/master/app/models/topic.rb#L911

https://github.com/discourse/discourse/blob/master/app/models/topic.rb#L823

Before we start to try to change the category, we should ensure that the tags requirement is fulfilled.

https://meta.discourse.org/t/the-category-setting-for-tags-is-not-respected/181214

lib/post_revisor.rb

@@ -75,11 +75,17 @@ class PostRevisor
     end
   end
 
-  track_topic_field(:category_id) do |tc, category_id|
+  track_topic_field(:category_id) do |tc, category_id, fields|
     if category_id == 0 && tc.topic.private_message?
       tc.record_change('category_id', tc.topic.category_id, nil)
       tc.topic.category_id = nil
     elsif category_id == 0 || tc.guardian.can_move_topic_to_category?(category_id)
+      tags = fields[:tags] || tc.topic.tags.map(&:name)
+      if category_id != 0 && !DiscourseTagging.validate_min_required_tags_for_category(tc.guardian, tc.topic, Category.find(category_id), tags)
+        tc.check_result(false)
+        next
+      end
+
       tc.record_change('category_id', tc.topic.category_id, category_id)
       tc.check_result(tc.topic.change_category_to_id(category_id))
     end
@@ -454,7 +460,7 @@ class PostRevisor
     Topic.transaction do
       PostRevisor.tracked_topic_fields.each do |f, cb|
         if !@topic_changes.errored? && @fields.has_key?(f)
-          cb.call(@topic_changes, @fields[f])
+          cb.call(@topic_changes, @fields[f], @fields)
         end
       end
 

spec/components/post_revisor_spec.rb

@@ -84,6 +84,28 @@ describe PostRevisor do
       post.revise(post.user, category_id: new_category.id)
       expect(post.reload.topic.category_id).to eq(new_category.id)
     end
+
+    it 'does not revise category if incorrect amount of tags' do
+      SiteSetting.min_trust_to_create_tag = 0
+      SiteSetting.min_trust_level_to_tag_topics = 0
+
+      new_category = Fabricate(:category, minimum_required_tags: 1)
+
+      post = create_post
+      old_category_id = post.topic.category_id
+
+      post.revise(post.user, category_id: new_category.id)
+      expect(post.reload.topic.category_id).to eq(old_category_id)
+
+      tag = Fabricate(:tag)
+      topic_tag = Fabricate(:topic_tag, topic: post.topic, tag: tag)
+      post.revise(post.user, category_id: new_category.id)
+      expect(post.reload.topic.category_id).to eq(new_category.id)
+      topic_tag.destroy
+
+      post.revise(post.user, category_id: new_category.id, tags: ['test_tag'])
+      expect(post.reload.topic.category_id).to eq(new_category.id)
+    end
   end
 
   context 'revise wiki' do