Rails Admin nested form example

example.rb

class Product < ActiveRecord::Base

  has_many :material_product_mappings, :dependent => :destroy, :inverse_of => :product
  has_many :materials, :through => :material_product_mappings, :autosave => true
  accepts_nested_attributes_for :material_product_mappings # really needed feature
  #...



end

class MaterialProductMapping < ActiveRecord::Base
  belongs_to :product, :inverse_of => :material_product_mappings
  belongs_to :material, :inverse_of => :material_product_mappings
  
  validates :num_units,
  :numericality =>  { :greater_than_or_equal_to => 0 }

  def name
    "#{self.material.name} (#{self.material.unit.name}) x #{self.num_units}"
  end
end
# file .../config/initializers/rails_admin.rb
  RailsAdmin.config do |config|
  config.model MaterialProductMapping do
    edit do
      form_builder :nested_form_for
      field :material_product_mappings do
        label "Materials"
        partial "edit_material_product_mappings"
      end
      #...
    end
  end
  end

# file .../app/views/rails_admin/main/_edit_material_product_mappings.html.erb

<div class="field <%= field.dom_id %>">
  <% _saved_object = @object %>
  <% _saved_form = form %>
  <% _saved_field = field %>
  <%# add here basic element's fileds if you have them %>
  <%= label_tag(field.dom_id, field.label) %>
  <div style="clear:left;padding-left:100px">
    <%= form.fields_for(:material_product_mappings) %>
    <p>
      <%= form.link_to_add('Add Material ', :material_product_mappings) %>
    </p>
    <% @object = _saved_object %>
    <% form = _saved_form %>
    <% field = _saved_field %>
    <% if field.has_errors? %>
      <span class="errorMessage"><%= "#{field.label } #{field.errors.first}" %></span>
    <% end %>
  </div>
</div>

# file .../app/views/rails_admin/main/_material_product_mapping_fields.html.erb
#  nested form looks for this file...
<% ff = f %>
<% ff.object.errors.full_messages.each do |msg| %>
   <p class="flash_alert_msg"><%= msg %></p>
<% end %>
<%= ff.number_field(:num_units, :min => 0.0, :step => 0.01, :size => 6, :style => "text-align:right") %>
<%= ff.collection_select('material_id',Material.active,:id,:name,:prompt => false) %>
<%= ff.link_to_remove('Remove') %>

I also had to override javascripts to make nested resources working. I hope that this will be fixed in future releases
# file .../app/assets/javascripts/rails_admin/rails_admin.js
//= require rails_admin/jquery-1.6.2.js
//= require rails_admin/jquery_ujs.js
//= require rails_admin/jquery-ui-1.8.14.js
//= require rails_admin/jquery.ui.timepicker.js
//= require rails_admin/ra.datetimepicker.js
//= require rails_admin/ra.filter-box.js
//= require rails_admin/activo/application.js
//= require rails_admin/activo/jquery.tipsy.js
//= require rails_admin/ra.filtering-multiselect.js
//= require rails_admin/ra.filtering-select.js
//= require rails_admin/ra.remote-form.js
//= require rails_admin/ra.timeline.js
//= require rails_admin/jquery.colorpicker.js
//= require jquery_nested_form
//= require rails_admin/application.js