Rails: Why "has_many ..., :through => ..." association results in "NameError: uninitialized constant ..."

To express that a group can have multiple users, and a user can belong to multiple groups, I set the following associations:

class Group < ActiveRecord::Base
  has_many :users_groups
  has_many :users, :through => :users_groups
end

class User < ActiveRecord::Base
  has_many :users_groups
  has_many :groups, :through => :users_groups
end

class UsersGroups < ActiveRecord::Base
  belongs_to :user
  belongs_to :group
end

However, when I type:

Group.find(1).users

I get:

NameError: uninitialized constant Group::UsersGroup

What am I doing wrong ?