FEATURE: new tags can be created from the "edit category" modal when defining the set of permitted tags

app/controllers/tags_controller.rb

@@ -112,17 +112,25 @@ class TagsController < ::ApplicationController
   end
 
   def search
-    query = DiscourseTagging.filter_allowed_tags(
+    category = params[:categoryId] ? Category.find_by_id(params[:categoryId]) : nil
+
+    tags_with_counts = DiscourseTagging.filter_allowed_tags(
       self.class.tags_by_count(guardian, params.slice(:limit)),
       guardian,
-      {
-        for_input: params[:filterForInput],
-        term: params[:q],
-        category: params[:categoryId] ? Category.find_by_id(params[:categoryId]) : nil
-      }
+      { for_input: params[:filterForInput], term: params[:q], category: category }
     )
 
-    tags = query.count.map {|t, c| { id: t, text: t, count: c } }
+    tags = tags_with_counts.count.map {|t, c| { id: t, text: t, count: c } }
+
+    unused_tags = DiscourseTagging.filter_allowed_tags(
+      Tag.where(topic_count: 0),
+      guardian,
+      { for_input: params[:filterForInput], term: params[:q], category: category }
+    )
+
+    unused_tags.each do |t|
+      tags << { id: t.name, text: t.name, count: 0 }
+    end
 
     render json: { results: tags }
   end

app/models/category.rb

@@ -318,6 +318,12 @@ SQL
   def allowed_tags=(tag_names)
     if self.tags.pluck(:name).sort != tag_names.sort
       self.tags = Tag.where(name: tag_names).all
+      if self.tags.size < tag_names.size
+        new_tag_names = tag_names - self.tags.map(&:name)
+        new_tag_names.each do |name|
+          self.tags << Tag.create(name: name)
+        end
+      end
     end
   end
 

spec/integration/category_tag_spec.rb

@@ -51,5 +51,10 @@ describe "category tag restrictions" do
       post = create_post(category: other_category, tags: [tag3.name, "newtag"])
       expect(post.topic.tags.map(&:name).sort).to eq([tag3.name, "newtag"].sort)
     end
+
+    it "can create tags when changing category settings" do
+      expect { other_category.update(allowed_tags: ['newtag']) }.to change { Tag.count }.by(1)
+      expect { other_category.update(allowed_tags: [tag1.name, 'tag-stuff', tag2.name, 'another-tag']) }.to change { Tag.count }.by(2)
+    end
   end
 end