Rails 3 has_many :through + join table conditions / scoping

Question

I'm working on an app that has the models User and Project, and User can be assigned to multiple Projects, via ProjectUser, with a role (e.g. Developer, Designer).

Project
  has_many :project_users
  has_many :users, :through => :project_users

User
  has_many :project_users
  has_many :projects, :through => :project_users

ProjectUser (user_id, project_id, role)
  belongs_to :user
  belongs_to :project

I can call @project.users and @user.projects, but since there are varying roles, I'd like to be a bit more specific with the relations. Ideally, I want to be able to do the following:

@project.developers
  # returns @project.users, but only where ProjectUser.role = 'Developer'

@project.designers << @user
  # creates a ProjectUser for @project, @user with role 'Designer'

@user.development_projects
  # returns projects where @user is assigned as a 'Developer'

@user.design_projects << @project
  # creates a ProjectUser for @project, @user with role 'Designer'

I currently have the following code:

has_many :developers, :through => :project_users, :source => :user,
                      :class_name => "User",
                      :conditions => ['project_users.role = ?','Developer']

But this only really does the fetching one-way, and doesn't give me much else - I can't build or assign or anything.

I'm attempting some more complex logic which I think might work, but would appreciate some pointers:

has_many :developer_assignments, :source => :project_user,
                                 :conditions => { :role => 'Developer' }
has_many :developers, :through => :developer_assignments # class_name?

Any suggestions? Thanks!

Answer 1

has_many accepts a block that can define/override methods for the association. This will allow you to create a custom method for <<. I've created a small example for you, you could create build in a similar fashion.

# Project.rb
has_many :developers, :through => :project_users, :source => :user,
         :conditions => "project_users.role = 'developer'" do
         def <<(developer)
           proxy_owner.project_users.create(:role => 'developer', :user => developer)
         end
       end

Now you can add a new developer to your your project with: @project.developers << @user as requested. @project.developers gives you all the developers.

If you have a lot of roles, it might be useful to create these has_many statements dynamically.

# Project.rb
ROLES = ['developer','contractor']

ROLES.each do |role|         
  self.class_eval <<-eos
    has_many :#{role.downcase}s, :through => :project_users, :source => :user,
           :conditions => "project_users.role = '#{role}'" do
             def <<(user)
               proxy_owner.project_users.create(:role => '#{role}', :user => user)
             end
           end
  eos
end

Looking back at everything above it doesn't seem like the rails way of doing things. Scoping this should make it possible to get the build and create commands working without redefining everything.

Hope this helps!

Answer 2

It sounds like what you're looking for is a combination of RoR's single table inheritance and named scopes.

Take a look at the following article for a nice example about polymorphic associations. This should help you with achieving the following:

@project.developers
  # returns @project.users, but only where ProjectUser.role = 'Developer'

@project.designers << @user
  # creates a ProjectUser for @project, @user with role 'Designer'

Scopes will give you a clean way to implement @user.development_projects but there may be more trickery required to get the << operator.