feat: ensure the packageManager field is set when installing

lib/install/template.rb

@@ -59,17 +59,28 @@ def package_json
   babel["presets"] ||= []
   babel["presets"].push("./node_modules/shakapacker/package/babel/preset.js")
 
+  package_manager = pj.fetch("packageManager", nil)
+
+  if package_manager.nil?
+    manager_checker = Shakapacker::ManagerChecker.new
+
+    package_manager = "#{manager_checker.guess_manager}@#{manager_checker.guess_manager_version}"
+  end
+
   {
     "name" => "app",
     "private" => true,
     "version" => "0.1.0",
     "babel" => babel,
     "browserslist" => [
       "defaults"
-    ]
+    ],
+    "packageManager" => package_manager
   }.merge(pj)
 end
 
+Shakapacker::ManagerChecker.new.warn_unless_package_manager_is_obvious!
+
 # Ensure there is `system!("bin/yarn")` command in `./bin/setup` file
 if (setup_path = Rails.root.join("bin/setup")).exist?
   native_install_command = package_json.manager.native_install_command.join(" ")

spec/generator_specs/generator_spec.rb

@@ -35,7 +35,14 @@
     # TODO: ideally "yarn_berry" should be here too, but it requires more complex setup
     NODE_PACKAGE_MANAGERS.reject { |fm| fm == "yarn_berry" }.each do |fallback_manager|
       context "when using package_json with #{fallback_manager} as the manager" do
+        let(:fake_manager_checker) { instance_double(Shakapacker::ManagerChecker).as_null_object }
+
         before :all do
+          allow(Shakapacker::ManagerChecker).to receive(:new).and_return(fake_manager_checker)
+
+          allow(fake_manager_checker).to receive(:guess_manager).and_return(:yarn)
+          allow(fake_manager_checker).to receive(:guess_manager_version).and_return("1.2.3")
+
           sh_opts = { fallback_manager: fallback_manager }
 
           sh_in_dir(sh_opts, SPEC_PATH, "cp -r '#{BASE_RAILS_APP_PATH}' '#{TEMP_RAILS_APP_PATH}'")
@@ -58,10 +65,13 @@
           expect(actual_content).to eq expected_content
         end
 
-        it "replaces the package.json completely" do
+        it "ensures the 'packageManager' field is set" do
           package_json = PackageJson.read(path_in_the_app)
 
-          expect(package_json.fetch("name", "")).to eq("app")
+          expect(package_json.fetch("packageManager", "")).to eq("[email protected]")
+
+          expect(fake_manager_checker).to have_received(:guess_manager)
+          expect(fake_manager_checker).to have_received(:guess_manager_version)
         end
 
         it "creates webpack config directory and its files" do