RoR "Has many" link > How to get title via foreign key?

I have a basic question of relationship in RoR :

In a view, I try to display the t开发者_C百科itle of a group, which is in the table "groups", and only the key "group_id" is stored in the table/object "product".

When I write product.group_id, I see the value in the database, but if I write product.group.title, RoR tolds me undefined method `group' for #.

This is a basic question so I appreciate your help !

The code in the view (this works but is so awful ! I'm sure there is a way like product.group.title)

<% @products.each do |product| %>
  <%= Group.find(product.group_id).title %>  </td>
<% end %>

and the models :

class Group < ActiveRecord::Base
  has_many :products, :dependent => :destroy
end

class Product < ActiveRecord::Base
  belongs_to :groups
  has_and_belongs_to_many :authors
end

---

Should be belongs_to :group not groups

Also, in your view, you could more easily do this:

<% @products.each do |product| %>
  <%= product.group.title %>  </td>
<% end %>