Add `without_projects` parameter to Topics API

Changelog: added

app/finders/projects/topics_finder.rb

@@ -13,6 +13,7 @@ def initialize(params: {})
 
     def execute
       topics = Projects::Topic.order_by_non_private_projects_count
+      topics = by_without_projects(topics)
       by_search(topics)
     end
 
@@ -25,5 +26,11 @@ def by_search(topics)
 
       topics.search(params[:search]).reorder_by_similarity(params[:search])
     end
+
+    def by_without_projects(topics)
+      return topics unless params[:without_projects]
+
+      topics.without_assigned_projects
+    end
   end
 end

app/models/projects/topic.rb

@@ -15,6 +15,7 @@ class Topic < ApplicationRecord
     has_many :project_topics, class_name: 'Projects::ProjectTopic'
     has_many :projects, through: :project_topics
 
+    scope :without_assigned_projects, -> { where(total_projects_count: 0) }
     scope :order_by_non_private_projects_count, -> { order(non_private_projects_count: :desc).order(id: :asc) }
     scope :reorder_by_similarity, -> (search) do
       order_expression = Gitlab::Database::SimilarityScore.build_expression(search: search, rules: [

doc/api/topics.md

@@ -20,11 +20,12 @@ GET /topics
 
 Supported attributes:
 
-| Attribute  | Type    | Required               | Description |
-| ---------- | ------- | ---------------------- | ----------- |
-| `page`     | integer | **{dotted-circle}** No | Page to retrieve. Defaults to `1`.                      |
-| `per_page` | integer | **{dotted-circle}** No | Number of records to return per page. Defaults to `20`. |
-| `search`   | string  | **{dotted-circle}** No | Search topics against their `name`.                     |
+| Attribute          | Type    | Required               | Description |
+| ------------------ | ------- | ---------------------- | ----------- |
+| `page`             | integer | **{dotted-circle}** No | Page to retrieve. Defaults to `1`.                      |
+| `per_page`         | integer | **{dotted-circle}** No | Number of records to return per page. Defaults to `20`. |
+| `search`           | string  | **{dotted-circle}** No | Search topics against their `name`.                     |
+| `without_projects` | boolean | **{dotted-circle}** No | Limit results to topics without assigned projects.      |
 
 Example request:
 

lib/api/topics.rb

@@ -12,6 +12,7 @@ class Topics < ::API::Base
     end
     params do
       optional :search, type: String, desc: 'Return list of topics matching the search criteria'
+      optional :without_projects, type: Boolean, desc: 'Return list of topics without assigned projects'
       use :pagination
     end
     get 'topics' do

spec/finders/projects/topics_finder_spec.rb

@@ -5,13 +5,13 @@
 RSpec.describe Projects::TopicsFinder do
   let_it_be(:user) { create(:user) }
 
-  let!(:topic1) { create(:topic, name: 'topicB') }
-  let!(:topic2) { create(:topic, name: 'topicC') }
-  let!(:topic3) { create(:topic, name: 'topicA') }
+  let_it_be(:topic1) { create(:topic, name: 'topicB') }
+  let_it_be(:topic2) { create(:topic, name: 'topicC') }
+  let_it_be(:topic3) { create(:topic, name: 'topicA') }
 
-  let!(:project1) { create(:project, :public, namespace: user.namespace, topic_list: 'topicC, topicA, topicB') }
-  let!(:project2) { create(:project, :public, namespace: user.namespace, topic_list: 'topicC, topicA') }
-  let!(:project3) { create(:project, :public, namespace: user.namespace, topic_list: 'topicC') }
+  let_it_be(:project1) { create(:project, :public, namespace: user.namespace, topic_list: 'topicC, topicA, topicB') }
+  let_it_be(:project2) { create(:project, :public, namespace: user.namespace, topic_list: 'topicC, topicA') }
+  let_it_be(:project3) { create(:project, :public, namespace: user.namespace, topic_list: 'topicC') }
 
   describe '#execute' do
     it 'returns topics' do
@@ -41,5 +41,21 @@
         end
       end
     end
+
+    context 'filter by without_projects' do
+      let_it_be(:topic4) { create(:topic, name: 'unassigned topic') }
+
+      it 'returns topics without assigned projects' do
+        topics = described_class.new(params: { without_projects: true }).execute
+
+        expect(topics).to contain_exactly(topic4)
+      end
+
+      it 'returns topics without assigned projects' do
+        topics = described_class.new(params: { without_projects: false }).execute
+
+        expect(topics).to contain_exactly(topic1, topic2, topic3, topic4)
+      end
+    end
   end
 end

spec/models/projects/topic_spec.rb

@@ -30,6 +30,17 @@
   end
 
   describe 'scopes' do
+    describe 'without_assigned_projects' do
+      let_it_be(:unassigned_topic) { create(:topic, name: 'unassigned topic') }
+      let_it_be(:project) { create(:project, :public, topic_list: 'topic') }
+
+      it 'returns topics without assigned projects' do
+        topics = described_class.without_assigned_projects
+
+        expect(topics).to contain_exactly(unassigned_topic)
+      end
+    end
+
     describe 'order_by_non_private_projects_count' do
       let!(:topic1) { create(:topic, name: 'topicB') }
       let!(:topic2) { create(:topic, name: 'topicC') }

spec/requests/api/topics_spec.rb

@@ -36,6 +36,22 @@
       expect(json_response[2]['total_projects_count']).to eq(1)
     end
 
+    context 'with without_projects' do
+      let_it_be(:topic_4) { create(:topic, name: 'unassigned topic', total_projects_count: 0) }
+
+      it 'returns topics without assigned projects' do
+        get api('/topics'), params: { without_projects: true }
+
+        expect(json_response.map { |t| t['id'] }).to contain_exactly(topic_4.id)
+      end
+
+      it 'returns topics without assigned projects' do
+        get api('/topics'), params: { without_projects: false }
+
+        expect(json_response.map { |t| t['id'] }).to contain_exactly(topic_1.id, topic_2.id, topic_3.id, topic_4.id)
+      end
+    end
+
     context 'with search' do
       using RSpec::Parameterized::TableSyntax