How create new instance of a model in relation many to many? - ruby-on-rails

Question

I try to create a furniture object, which is in relation by a has_many_and_belongs_to with stores, this is my model:
class Furniture < ActiveRecord::Base
attr_accessible :area, :description, :name, :size
has_and_belongs_to_many :stores
end
My problem is that I don't know how create a new furniture, because i try to associate furniture with one or more store with check box, but I obtain this error: undefined method merge for #<Store:0x007ff16ae27e40>.
These are my view with form and my controller with new and create action:
View:
<%= form_for #furniture do |f| %>
<%= f.label :name %>
<%= f.text_field :name %> <br><br>
<%= f.label :description %>
<%= f.text_field :description %> <br><br>
<%= f.label :size %>
<%= f.text_field :size %> <br><br>
<% #store.each do |store| %>
<div>
<%= f.check_box :stores, store %>
<%= store.name %>
</div>
<% end %>
<%= f.submit %>
<% end %>
Controller:
def new
#furniture = Furniture.new
#store = Store.order('name ASC')
end
def create
#furniture = Furniture.create(params[:furniture])
redirect_to admins_path
end
How can I solve it?? Have you some suggestion to create a new object with this relation ship??
Thank you very much
EDIT:
I have a join table between furniture and store

Answer 1

The has_and_belongs_to association adds a method collection_singular_ids= that for the current case will be #furniture.store_ids=. According to the docs
The collection_singular_ids= method makes the collection contain only the objects identified by the supplied primary key values, by adding and deleting as appropriate.
So, you can use this idea to add the stores to your furniture. Replace
<% #store.each do |store| %>
<div>
<%= f.check_box :stores, store %>
<%= store.name %>
</div>
<% end %>
with
<% #store.each do |store| %>
<div>
<%= f.check_box :store_ids, {:multiple => true}, store.id, nil %>
<%= store.name %>
</div>
<% end %>