Generate empty yarn.lock using Yarn on Heroku

Closes [#538].

Generate an empty `yarn.lock` so that Heroku understands that the
application's deployment target requires `yarn`.

[#538]: #538

CHANGELOG.md

@@ -1,6 +1,12 @@
 master
 ------
 
+* Generate an empty `yarn.lock` so that Heroku understands that the
+  application's deployment target requires `yarn`. Closes [#538]. [#540]
+
+[#538]: https://github.com/thoughtbot/ember-cli-rails/issues/538
+[#540]: https://github.com/thoughtbot/ember-cli-rails/pull/540
+
 0.9.0
 -----
 

ember-cli-rails.gemspec

@@ -19,4 +19,5 @@ Gem::Specification.new do |spec|
   spec.add_dependency "railties", ">= 3.2"
   spec.add_dependency "cocaine", "~> 0.5.8"
   spec.add_dependency "html_page", "~> 0.1.0"
+  spec.add_development_dependency "generator_spec", "~> 0.9.0"
 end

lib/ember_cli/app.rb

@@ -83,6 +83,10 @@ def mountable?
       deploy.mountable?
     end
 
+    def yarn_enabled?
+      options.fetch(:yarn, false)
+    end
+
     def to_rack
       deploy.to_rack
     end

lib/generators/ember/heroku/heroku_generator.rb

@@ -13,6 +13,12 @@ def copy_setup_heroku_file
       run "chmod a+x bin/heroku_install"
     end
 
+    def identify_as_yarn_project
+      if EmberCli.apps.values.any?(&:yarn_enabled?)
+        template "yarn.lock.erb", "yarn.lock"
+      end
+    end
+
     def inject_12factor_gem
       gem "rails_12factor", group: [:staging, :production]
     end

lib/generators/ember/heroku/templates/yarn.lock.erb

@@ -0,0 +1 @@
+

spec/generators/ember/heroku/heroku_generator_spec.rb

@@ -0,0 +1,40 @@
+require "rails/generators"
+require "generator_spec"
+require "generators/ember/heroku/heroku_generator"
+
+describe EmberCli::HerokuGenerator, type: :generator do
+  destination Rails.root.join("tmp", "generator_test_output")
+
+  before :all do
+    prepare_destination
+    FileUtils.touch(destination_root.join("Gemfile"))
+  end
+
+  context "without yarn enabled" do
+    it "does not generate a root-level yarn.lock" do
+      EmberCli.configure do |c|
+        c.app "my-app", yarn: false
+      end
+
+      run_generator
+
+      expect(destination_root).to have_structure {
+        no_file "yarn.lock"
+      }
+    end
+  end
+
+  context "with yarn enabled" do
+    it "generates a root-level yarn.lock" do
+      EmberCli.configure do |c|
+        c.app "my-app", yarn: true
+      end
+
+      run_generator
+
+      expect(destination_root).to have_structure {
+        file "yarn.lock"
+      }
+    end
+  end
+end

spec/lib/ember_cli/app_spec.rb

@@ -25,6 +25,28 @@
     end
   end
 
+  describe "yarn_enabled?" do
+    context "when configured with yarn: true" do
+      it "returns true" do
+        app = EmberCli::App.new("with-yarn", yarn: true)
+
+        yarn_enabled = app.yarn_enabled?
+
+        expect(yarn_enabled).to be true
+      end
+    end
+
+    context "when configured with yarn: false" do
+      it "returns false" do
+        app = EmberCli::App.new("without-yarn", yarn: false)
+
+        yarn_enabled = app.yarn_enabled?
+
+        expect(yarn_enabled).to be false
+      end
+    end
+  end
+
   describe "#compile" do
     it "exits with exit status of 0" do
       passed = EmberCli["my-app"].compile