undefined method `has_many' for Formtastic - ruby-on-rails-3

Question

I'm getting this error :
undefined method `has_many' for #<Formtastic::SemanticFormBuilder:0xb410d4c>
It work when I use it like this :
ActiveAdmin.register Ressource do
form do |f|
f.inputs do
f.input :offer_id, :as => :hidden
f.input :name
f.input :category, :include_blank => false, :collection => Category.order('place ASC').all, :member_label => :to_label
f.input :description, :input_html => {:class => 'editor'}
f.input :price
end
f.has_many :roles do |app_f|
app_f.inputs do
if not app_f.object.id.nil?
app_f.input :_destroy, :as => :boolean, :label => "Supprimer l'utilisateur du lot"
end
app_f.input :user, :member_label => :to_label, :label => 'Assigné le lot'
app_f.input :name, :include_blank => false
end
end
f.buttons
end
end
But it doesn't work in a partial, i need to call the has_many method by a different way or something else ?

Answer 1

ActiveAdmin extends formtastic with some useful helpers such as has_many (lib/active_admin/form_builder.rb in the activeadmin gem).
Unfortunately, these helpers are not available by default in your templates.
Here are two options:
If you don't need the extra has_many functionality (it looks like active_admin adds some javascript to make it easy to add a new record to the collection), then you can use stock formtastic. This example should work fine in the activeadmin file as well as in a partial:
ActiveAdmin.register Ressource do
form do |f|
# ...
f.inputs :for => :roles do |app_f|
# ...
app_f.input :name, :include_blank => false
end
f.buttons
end
end
Use the ActiveAdmin form builder explicitly:
<%= semantic_form_for [:admin, #resource], builder: ActiveAdmin::FormBuilder do |f| %>
<!-- ... -->
<%= f.has_many :teachers do |app_f| %>
<%= app_f.inputs do %>
<!-- ... -->
<% end %>
<% end %>
<%= f.buttons %>
<% end %>
I hope this helps.

Answer 2

There is a solution
form :html => {:multipart => true} do |f|
end
Or, if you want to use partial:
<%= active_admin_form_for [:admin, #resource] ,:html => {:multipart => true} do |f|%>
<% end %>