Add associated object generator

README.md

@@ -85,6 +85,12 @@ class Post::Publisher < ActiveRecord::AssociatedObject
 end
 ```
 
+### Use the generator to help write Associated Objects
+
+To set up the `Post::Publisher` from above, you can call `bin/rails generate associated Post::Publisher`.
+
+See `bin/rails generate associated --help` for more info.
+
 ### Forwarding callbacks onto the associated object
 
 To further help illustrate how your collaborator Associated Objects interact with your domain model, you can forward callbacks.

lib/generators/associated/USAGE

@@ -0,0 +1,15 @@
+Description:
+    Create a PORO collaborator associated object inheriting from `ActiveRecord::AssociatedObject` that's associated with an Active Record record class.
+
+    It'll be associated on the record with `has_object`.
+
+    Note: associated object names support pluralized class names. So "Seats" remain "seats" in all cases, and "Seat" remains "seat" in all cases.
+Example:
+    bin/rails generate associated Organization::Seats
+
+    This will create:
+        app/models/organization/seats.rb
+        test/models/organization/seats_test.rb
+
+    And in Organization, this will insert:
+      has_object :seats

lib/generators/associated/associated_generator.rb

@@ -0,0 +1,27 @@
+class AssociatedGenerator < Rails::Generators::NamedBase
+  source_root File.expand_path("templates", __dir__)
+
+  def generate_associated_object_files
+    template "associated.rb",      "app/models/#{name.underscore}.rb"
+    template "associated_test.rb", "test/models/#{name.underscore}_test.rb"
+  end
+
+  def connect_associated_object
+    record_file = "#{destination_root}/app/models/#{record_path}.rb"
+    raise "Record class '#{record_klass}' does not exist" unless File.exist?(record_file)
+
+    inject_into_class record_file, record_klass do
+      optimize_indentation "has_object :#{associated_object_path}", 2
+    end
+  end
+
+  private
+
+  # The `:name` argument can handle model names, but associated object class names aren't singularized.
+  # So these record and associated_object methods prevent that.
+  def record_path  = record_klass.downcase.underscore
+  def record_klass = name.deconstantize
+
+  def associated_object_path  = associated_object_class.downcase.underscore
+  def associated_object_class = name.demodulize
+end

lib/generators/associated/templates/associated.rb.tt

@@ -0,0 +1,5 @@
+class <%= name %> < ActiveRecord::AssociatedObject
+  extension do
+    # Extend <%= record_klass %> here
+  end
+end

lib/generators/associated/templates/associated_test.rb.tt

@@ -0,0 +1,8 @@
+require "test_helper"
+
+class <%= name %>Test < ActiveSupport::TestCase
+  setup do
+    # @<%= record_path %> = <%= record_path.pluralize %>(:TODO_fixture_name)
+    # @<%= associated_object_path %> = @<%= record_path %>.<%= associated_object_path %>
+  end
+end

test/boot/active_record.rb

@@ -1,5 +1,5 @@
 ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
-ActiveRecord::Base.logger = Logger.new(STDOUT)
+ActiveRecord::Base.logger = Logger.new(STDOUT) if ENV["VERBOSE"] || ENV["CI"]
 
 ActiveRecord::Schema.define do
   create_table :authors, force: true do |t|

test/lib/generators/associated_generator_test.rb

@@ -0,0 +1,72 @@
+require "test_helper"
+require "pathname"
+require "rails/generators"
+require "generators/associated/associated_generator"
+
+class AssociatedGeneratorTest < Rails::Generators::TestCase
+  tests AssociatedGenerator
+  destination Pathname(__dir__).join("../../../tmp/generators")
+
+  setup :prepare_destination, :create_record_file, :create_record_test_file
+  arguments %w[Organization::Seats]
+
+  test "generator runs without errors" do
+    assert_nothing_raised { run_generator }
+  end
+
+  test "generates an object.rb file" do
+    run_generator
+
+    assert_file "app/models/organization/seats.rb", <<~RUBY
+      class Organization::Seats < ActiveRecord::AssociatedObject
+        extension do
+          # Extend Organization here
+        end
+      end
+    RUBY
+  end
+
+  test "generates an object_test.rb file" do
+    run_generator
+
+    assert_file "test/models/organization/seats_test.rb", /Organization::SeatsTest/
+  end
+
+  test "connects record" do
+    run_generator
+
+    assert_file "app/models/organization.rb", <<~RUBY
+      class Organization
+        has_object :seats
+      end
+    RUBY
+  end
+
+  test "raises error if associated record doesn't exist" do
+    assert_raise RuntimeError do
+      run_generator ["Business::Monkey"]
+    end
+  end
+
+  private
+
+  def create_record_file
+    create_file "app/models/organization.rb", <<~RUBY
+      class Organization
+      end
+    RUBY
+  end
+
+  def create_record_test_file
+    create_file "test/models/organization_test.rb", <<~RUBY
+      require "test_helper"
+
+      class OrganizationTest < ActiveSupport::TestCase
+      end
+    RUBY
+  end
+
+  def create_file(path, content)
+    destination_root.join(path).tap { _1.dirname.mkpath }.write content
+  end
+end