Nested Models

Nested Models

Nested Models let you build forms that update an object and its associated objects all in one shot.

(note this is a work in progress!)

Background

To use nested models with simple_form, there are two rails features that you have to understand:

• accepts_nested_attributes_for - an ActiveRecord class method that goes in your model code - it lets you create and update child objects through the associated parent. An in depth explanation is available in the ActiveRecord documentation.
• fields_for - a helper method that goes in your view code - it builds form fields for your nested models

TBD - format this text and add some reference links

An example

Let's take a simple example:

    class Machine < ActiveRecord::Base
      has_many :parts
      accepts_nested_attributes_for :parts
    end

    class Part < ActiveRecord::Base
      # name:string
      belongs_to :machine
    end

With these models, we can use simple_form to update the machine and its associated parts in a single form:

    <%= simple_form_for @machine do |m| %>
      <%= m.simple_fields_for :parts do |p| %>
        <%= p.input :name %>
      <% end %>
    <% end %>

For 'new' action, build the nested model from the controller:

  def new
    @machine = Machine.new
    @machine.parts.build
  end

In case of 1-1 relationship

e.g. if Machine has_one Color, accepts_nested_attributes_for must be called with :color.

accepts_nested_attributes_for :color

And in your Controller#new action, you should make a new instance for the :color attribute:

  def new
    @machine = Machine.new
    @machine.color = Color.new
  end

If you have non-ActiveRecord models, then refer to Use with ActiveModel compliant models page.

Example for key-value associations

To fill the label of the value field with the key attribute and give it an ID so we can grab it with Capybara, we can access the object in question by calling nested_form.object

    <%= simple_form_for @user do |m| %>
      <%= m.simple_fields_for :custom_fields do |c| %>
        <%= c.input :field_key, as: :hidden %>
        <%= c.input :field_value, label: c.object.field_key, input_html: { id: c.object.field_key } %>
      <% end %>
    <% end %>