Only set MR description from template when feature available

Only when the Issuable Default Templates feature is available, fill in
the description from settings when building a new MR.

app/services/ee/merge_requests/build_service.rb

+module EE
+  module MergeRequests
+    module BuildService
+      def assign_title_and_description
+        super
+
+        assign_description_from_template
+      end
+
+      # Set MR description based on project template
+      def assign_description_from_template
+        return unless target_project.feature_available?(:issuable_default_templates) &&
+            target_project.merge_requests_template.present?
+
+        merge_request.description = target_project.merge_requests_template
+
+        append_closes_description
+      end
+    end
+  end
+end

app/services/merge_requests/build_service.rb

 module MergeRequests
   class BuildService < MergeRequests::BaseService
+    prepend EE::MergeRequests::BuildService
+
     def execute
       self.merge_request = MergeRequest.new(params)
       merge_request.compare_commits = []
@@ -105,42 +107,60 @@ module MergeRequests
     #   more than one commit in the MR
     #
     def assign_title_and_description
-      if match = source_branch.match(/\A(\d+)-/)
-        iid = match[1]
-      end
+      assign_title_and_description_from_single_commit
+
+      assign_title_from_issue
 
+      merge_request.title ||= source_branch.titleize.humanize
+
+      merge_request.title = wip_title if compare_commits.empty?
+
+      append_closes_description
+    end
+
+    def assign_title_and_description_from_single_commit
       commits = compare_commits
-      if commits && commits.count == 1
-        commit = commits.first
-        merge_request.title = commit.title
-        merge_request.description ||= commit.description.try(:strip)
-      elsif iid && issue = target_project.get_issue(iid, current_user)
-        case issue
-        when Issue
-          merge_request.title = "Resolve \"#{issue.title}\""
-        when ExternalIssue
-          merge_request.title = "Resolve #{issue.title}"
-        end
-      else
-        merge_request.title = source_branch.titleize.humanize
-      end
 
-      # Set MR description based on project template
-      if merge_request.target_project.merge_requests_template.present?
-        merge_request.description = merge_request.target_project.merge_requests_template
+      return unless commits && commits.count == 1
+
+      commit = commits.first
+      merge_request.title ||= commit.title
+      merge_request.description ||= commit.description.try(:strip)
+    end
+
+    def assign_title_from_issue
+      return unless issue
+
+      case issue
+      when Issue
+        merge_request.title ||= "Resolve \"#{issue.title}\""
+      when ExternalIssue
+        merge_request.title ||= "Resolve #{issue.title}"
       end
+    end
 
-      if iid
-        closes_issue = "Closes ##{iid}"
+    def append_closes_description
+      return unless issue_iid
 
-        if description.present?
-          merge_request.description += closes_issue.prepend("\n\n")
-        else
-          merge_request.description = closes_issue
-        end
+      closes_issue = "Closes ##{issue_iid}"
+
+      if description.present?
+        merge_request.description += closes_issue.prepend("\n\n")
+      else
+        merge_request.description = closes_issue
       end
+    end
+
+    def issue_iid
+      return @issue_iid if defined?(@issue_iid)
+
+      @issue_iid = source_branch[/\A(\d+)-/, 1]
+    end
+
+    def issue
+      return @issue if defined?(@issue)
 
-      merge_request.title = wip_title if commits.empty?
+      @issue = target_project.get_issue(issue_iid, current_user)
     end
   end
 end

spec/services/ee/merge_requests/build_service_spec.rb

+require 'spec_helper'
+
+describe MergeRequests::BuildService, services: true do # rubocop:disable RSpec/FilePath
+  let(:source_project) { project }
+  let(:target_project) { project }
+  let(:user) { create(:user) }
+  let(:description) { nil }
+  let(:source_branch) { 'feature-branch' }
+  let(:target_branch) { 'master' }
+  let(:merge_request) { service.execute }
+  let(:compare) { double(:compare, commits: commits) }
+  let(:commit_1) { double(:commit_1, safe_message: "Initial commit\n\nCreate the app") }
+  let(:commit_2) { double(:commit_2, safe_message: 'This is a bad commit message!') }
+  let(:commits) { nil }
+
+  let(:service) do
+    MergeRequests::BuildService.new(project, user,
+                                    description: description,
+                                    source_branch: source_branch,
+                                    target_branch: target_branch,
+                                    source_project: source_project,
+                                    target_project: target_project)
+  end
+
+  before do
+    allow(service).to receive(:branches_valid?) { true }
+  end
+
+  context 'project default template configured' do
+    let(:template) { "I am the template, you fill me in" }
+    let(:project) { create(:empty_project, merge_requests_template: template) }
+
+    context 'issuable default templates feature not available' do
+      before do
+        stub_licensed_features(issuable_default_templates: false)
+      end
+
+      it 'does not set the MR description from template' do
+        expect(merge_request.description).not_to eq(template)
+      end
+    end
+
+    context 'issuable default templates feature available' do
+      before do
+        stub_licensed_features(issuable_default_templates: true)
+      end
+
+      it 'sets the MR description from template' do
+        expect(merge_request.description).to eq(template)
+      end
+    end
+  end
+end