Is it possible to create a conditional association in model?

Question

I have setup a role based access controll system with the following models:

• Role (as STI),
  • UserRole (global roles)
  • ProjectRole (project specific roles)
• Assignment (Polymorphic with different resources)
• User
• Project (as one resource type for assignments)

Users are only allowed to be responsible for a project if they have a specific UserRole. This Userrole is name "responsible for projects" and has ID 2.

In User model there are two has_many associations :responsible_assignments and responsible_projects. This associations are only valid if the user has the UserRole "responsible for projects" with ID 2.

Is it possible to create a conditional association in user model for responsible_* association and is this a common way to setup this kind of relations?

What is the best practise to solve this kind of problems?

class Role < ActiveRecord::Base
  has_many :assignments
  has_many :users, :through => :assignments

class UserRole < Role

class ProjectRole < Role

class Assignment < ActiveRecord::Base
  belongs_to :user
  belongs_to :role
  belongs_to :resource, :polymorphic => true

class User < ActiveRecord::Base
  has_many :assignments
  has_many :roles, :through => :assignments, 
                   :class_name => "UserRole"
  has_many :responsible_assignments, :class_name => "Assignment",
                                     :conditions => { :role_id => 4 }     // specific project role
  has_many :responsible_projects, :through => :responsible_assignments, 
                                 :source => :resource, 
                                 :source_type => 'Project',
                                 :conditions => { :status => 1 }          // project is active
  ...

class Project < ActiveRecord
  ...

Answer 1

In case anyone finds this later - this feature is now actually available in rails 4:

http://guides.rubyonrails.org/association_basics.html

Syntax is:

has_many :orders, -> { where processed: true }