My has_many :through association of Users, Groups, and Memberships is causing trouble in some methods

I am working with a has_many :through association: Users join Groups through a Membership. I am having trouble with some methods I created in the user model to determine if the user is a member of a group, and to 开发者_运维知识库allow a user to join and leave a group. Using the console, memberships.find_by_group_id always returns nil. I am not sure why and I think it might be the way I have set up my has_many :through associations, though after looking it over many, many times and consulting railscasts/blogs it seems ok. If you want me to post more info, like the schema.db or something, let me know

class User < ActiveRecord::Base
 has_many :memberships, :dependent => :destroy
 has_many :groups, :through => :memberships
.
.
.
  def member?(group)
    memberships.find_by_group_id(group)
  end

  def join!(group)
    memberships.create!(:group_id => group.id)
  end

  def leave!(group)
    memberships.find_by_group_id(group).destroy
  end
.
.
.
end

class Group < ActiveRecord::Base
 has_many :memberships
 has_many :members, :through => :memberships, :source => :user
 has_attached_file :photo, :styles => { :thumb => "100x100",
                                        :small => "200x200" }
 attr_accessible :name, :description, :private, :created_at, :group_id
 attr_accessible :photo, :photo_file_name, :photo_content_type,
                 :photo_file_size,  :photo_updated_at                                  
end

class Membership < ActiveRecord::Base
 attr_accessible :group_id

 belongs_to :user
 belongs_to :group
end

Here is the membership controller:

class MembershipsController < ApplicationController
def create
  @group = Group.find(params[:membership][:group_id])
  current_user.join!(@group)
  respond_to do |format|
    format.html { redirect_to @group }
    format.js
  end
end

def destroy
   @group = Membership.find(params[:id]).group
   current_user.leave!(@group)
   respond_to do |format|
     format.html { redirect_to @group }
     format.js
   end
end

def index
  @memberships = Membership.all
end

end

I think what you are looking for is :has_and_belongs_to_many (N-N) association rather than :has_many (1-N) association.

It seems that each group can have many users as well.

Take a look at official rails guides at http://guides.rubyonrails.org/association_basics.html#has_and_belongs_to_many-association-reference

Using :has_and_belongs_to_many relationships, it will be really easy to add a collection(user) to the other collection(membership) .

For example

@membership.users << current_user

will add current_user to the membership

and

@membership.users.delete(current_user)

will delete current_user from the membership.