Rails 3, how to make sure Group has_one user.role = groupleader

My current Group model:

class Group < ActiveRecord::Base
   has_many :memberships, :dependent => :destroy
   has_many :users, :through => :memberships
end

My Current User Model

 class User < ActiveRecord::Base
      has_and_belongs_to_many :roles
      has_many :memberships, :dependent => :destroy
      has_many :groups, :through => :memberships
      #some more stuff
    end 

Membership Model

class Membership < ActiveRecord::Base
  attr_accessible :user_id, :group_id
  belongs_to :user
  belongs_to :group 
end

Role Model

class Role < ActiveRecord::Base
  has_and_belongs_to_many开发者_开发问答 :users
end

I have a Ability class and CanCan installed to handle roles. I have a role type groupleader and need to make sure a Group has only one groupleader...

I think its something like: Group has_one User.role :groupleader... but I know thats not it.

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It doesn't make sense to me to have the role on the users table if you want it to determine what the user can do within the context of a group.

Where it would make sense is to have it on the memberships table for groups and users. Records in this table would then have three columns: user_id, group_id and role.

Then to retrieve the leader for the group you would execute a query like this:

group.users.where("memberships.role = 'leader'").first

Where group is a Group object, i.e. Group.first or Group.find(13).

This then leaves open the possibility that you can have more than one leader for a group further down the track if required.

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If your roles are in a separate table, then you can do this:

group.users.where("memberships.role_id = ?", Role.find_by_name("leader").id).first