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

@@ -3,6 +3,7 @@
 require "json"
 require "shakapacker/utils/misc"
 require "shakapacker/utils/version_syntax_converter"
+require "shakapacker/manager_checker"
 require "package_json"
 
 GEM_ROOT = Pathname.new(File.expand_path("../../..", __FILE__))
@@ -58,10 +59,12 @@
           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")
+          manager_name = fallback_manager.split("_")[0]
+
+          expect(package_json.fetch("packageManager", "")).to match(/#{manager_name}@\d+\.\d+\.\d+/)
         end
 
         it "creates webpack config directory and its files" do