How to access the joining model on a has_many :through relationship

Question

I have a typical many-to-many relationship using has_many => :through, as detailed below.

class member
  has_many member_roles
  has_many roles, :through => :member_roles
end

class role
  has_many member_roles
  has_man members, :through => :member_roles
end

class member_role
  belongs_to :member
  belongs_to :role
  # has following fields: member_id, role_id, scope, sport_id
end

What I'm trying to do here is allow members to be assigned roles. Each member role has a scope, which by default is set to "All" but if desired can be set to "Sport". If the scope is set to sport then we also capture the sport_id, which allows us to restrict assess on that role to a particular sport (ie, can only manage the teams of that sport, rather than the teams of every sport). Sounds simple enough.

I have setup my update_member_roles action something like this:

def update

  # Assume we passing a param like: params[:member][:roles]
  # as an array of hashes consisting of :role_id and if set, :sport_id

  roles = (params[:member] ||= {}).delete "roles"
  @member.roles = Role.find_all_by_id(roles.map{|r| r["role_id"]})
  if @member.update_attributes params[:member]
    flash[:notice] = "Roles successfully updated."
    redirect_to member_path(@member)
  else
    render :action => "edit"
  end
end

The above works nice enough, it sets the appropriate member_roles very nicely... but as I'm working on the Role model and not the MemberRole model I'm kind of stuck as to how I can access the joining model to set the :scope and :sport_id.

Any pointers here would be much appreciated.

Answer 1

You should use the member_roles association instead of roles association.

  def update
    # The role hash should contain :role_id, :scope and if set, :sport_id
    roles = ((params[:member] ||= {}).delete "roles") || []
    MemberRole.transaction do 

      # Next line will associate the :sport_id and :scope to member_roles
      # EDIT: Changed the code to flush old roles.
      @member.member_roles = roles.collect{|r| MemberRole.new(r)}

      # Next line will save the member attributes and member_roles in one TX
      if @member.update_attributes params[:member]
        # some code
      else
        # some code
      end
    end
  end

Make sure you enclose everything in one transaction.

Other possibility is to use accepts_nested_attributes_for on member_roles association.

Answer 2

Sounds like you should look into using nested attributes:

• Railscast #196
• Railscast #197
• Rails API docs