Don't build project services unneccesarily

app/controllers/projects/merge_requests_controller.rb

@@ -217,7 +217,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
 
           metrics_url =
             if can?(current_user, :read_environment, environment) && environment.has_metrics?
-              metrics_project_environment_deployment_path(environment.project, environment, deployment)
+              metrics_project_environment_deployment_path(project, environment, deployment)
             end
 
           metrics_monitoring_url =

app/models/project.rb

@@ -1082,26 +1082,10 @@ class Project < ActiveRecord::Base
   end
 
   def find_or_initialize_services(exceptions: [])
-    services_templates = Service.where(template: true)
-
     available_services_names = Service.available_services_names - exceptions
 
     available_services = available_services_names.map do |service_name|
-      service = find_service(services, service_name)
-
-      if service
-        service
-      else
-        # We should check if template for the service exists
-        template = find_service(services_templates, service_name)
-
-        if template.nil?
-          # If no template, we should create an instance. Ex `build_gitlab_ci_service`
-          public_send("build_#{service_name}_service") # rubocop:disable GitlabSecurity/PublicSend
-        else
-          Service.build_from_template(id, template)
-        end
-      end
+      find_or_initialize_service(service_name)
     end
 
     available_services.reject do |service|
@@ -1114,7 +1098,18 @@ class Project < ActiveRecord::Base
   end
 
   def find_or_initialize_service(name)
-    find_or_initialize_services.find { |service| service.to_param == name }
+    service = find_service(services, name)
+    return service if service
+
+    # We should check if template for the service exists
+    template = find_service(services_templates, name)
+
+    if template
+      Service.build_from_template(id, template)
+    else
+      # If no template, we should create an instance. Ex `build_gitlab_ci_service`
+      public_send("build_#{name}_service") # rubocop:disable GitlabSecurity/PublicSend
+    end
   end
 
   # rubocop: disable CodeReuse/ServiceClass
@@ -2277,4 +2272,8 @@ class Project < ActiveRecord::Base
       check_access.call
     end
   end
+
+  def services_templates
+    @services_templates ||= Service.where(template: true)
+  end
 end

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -772,16 +772,26 @@ describe Projects::MergeRequestsController do
         create(:merge_request, source_project: forked, target_project: project)
       end
 
-      before do
+      it 'links to the environment on that project' do
+        get_ci_environments_status
+
+        expect(json_response.first['url']).to match /#{forked.full_path}/
+      end
+
+      # we're trying to reduce the overall number of queries for this method.
+      # set a hard limit for now. https://gitlab.com/gitlab-org/gitlab-ce/issues/43109
+      it 'keeps queries in check' do
+        control_count = ActiveRecord::QueryRecorder.new { get_ci_environments_status }.count
+
+        expect(control_count).to be <= 137
+      end
+
+      def get_ci_environments_status
         get :ci_environments_status,
           namespace_id: merge_request.project.namespace.to_param,
           project_id: merge_request.project,
           id: merge_request.iid, format: 'json'
       end
-
-      it 'links to the environment on that project' do
-        expect(json_response.first['url']).to match /#{forked.full_path}/
-      end
     end
   end
 

spec/models/project_spec.rb

@@ -4037,6 +4037,20 @@ describe Project do
         expect(result).to be_empty
       end
     end
+
+    describe "#find_or_initialize_service" do
+      subject { build(:project) }
+
+      it 'avoids N+1 database queries' do
+        allow(Service).to receive(:available_services_names).and_return(%w(prometheus pushover))
+
+        control_count = ActiveRecord::QueryRecorder.new { project.find_or_initialize_service('prometheus') }.count
+
+        allow(Service).to receive(:available_services_names).and_call_original
+
+        expect { project.find_or_initialize_service('prometheus') }.not_to exceed_query_limit(control_count)
+      end
+    end
   end
 
   def rugged_config