When you render a nested form for a Movie which has_many :actors, you want to render the right number of blank Actor forms. The right number means:

• A minimum number of blank forms so the user can add more Actors to an existing Movie, e.g. 2.
• A minimum total number of forms (both blank and pre-filled) so the user sees more than just 2 blank Actor forms when she is entering Actors for the first time, e.g. 5.

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For the example above, this is the desired progression of the number of blank forms:

Luckily, there's an app for that. Copy the attached file to config/initializers to get a handy method build_nested_records:

- form_for @movie do |form|
  = form.text_field :title
  - form.build_nested_records(:actors)
  - form.fields_for :actors do |actor_form|
    = actor_form.text_field :name

Options

The build_nested_records helper takes an options hash like this:

form.build_nested_records(:actors, minimum: 7, buffer: 3)

• The :minimum option is the minimum number of total records for which forms will be rendered. The default is 5.
• The :buffer option is the minimum number of blank forms you want. The default is 2.
• You can also give a :maximum option to limit the number of total rows. By default there is no limit.

You can also set defaults for built records by adding additional keys to the options hash:

form.build_nested_records(:actors, :minimum: 4, name: 'Foo', active: true)

If you want to call a different method rather than build, you can pass a block:

form.build_nested_records(:actors) { @record.my_custom_build_method }

Building has_one associations

When your Movie has_one :director, the initializer won't work because form.object.director is nil, if there's no director yet. Just use the default form.object.build_director. Generally spoken:

form.object.build_<attribute_name>