Disable_joins for Ci::Runner groups

As Ci::Runner is allowed only one group at most, via validation and
unique constraint this should be safe

app/models/ci/runner.rb

@@ -51,7 +51,7 @@ module Ci
     has_many :runner_projects, inverse_of: :runner, autosave: true, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
     has_many :projects, through: :runner_projects
     has_many :runner_namespaces, inverse_of: :runner, autosave: true
-    has_many :groups, through: :runner_namespaces
+    has_many :groups, through: :runner_namespaces, disable_joins: true
 
     has_one :last_build, -> { order('id DESC') }, class_name: 'Ci::Build'
 

spec/models/ci/runner_spec.rb

@@ -5,6 +5,20 @@ require 'spec_helper'
 RSpec.describe Ci::Runner do
   it_behaves_like 'having unique enum values'
 
+  describe 'groups association' do
+    # Due to other assoctions such as projects this whole spec is allowed to
+    # generate cross-database queries. So we have this temporary spec to
+    # validate that at least groups association does not generate cross-DB
+    # queries.
+    it 'does not create a cross-database query' do
+      runner = create(:ci_runner, :group)
+
+      with_cross_joins_prevented do
+        expect(runner.groups.count).to eq(1)
+      end
+    end
+  end
+
   describe 'validation' do
     it { is_expected.to validate_presence_of(:access_level) }
     it { is_expected.to validate_presence_of(:runner_type) }

spec/requests/api/ci/runners_spec.rb

@@ -291,6 +291,16 @@ RSpec.describe API::Ci::Runners do
       end
     end
 
+    context 'when the runner is a group runner' do
+      it "returns the runner's details" do
+        get api("/runners/#{group_runner_a.id}", admin)
+
+        expect(response).to have_gitlab_http_status(:ok)
+        expect(json_response['description']).to eq(group_runner_a.description)
+        expect(json_response['groups'].first['id']).to eq(group.id)
+      end
+    end
+
     context "runner project's administrative user" do
       context 'when runner is not shared' do
         it "returns runner's details" do