account.rb

class Account < ActiveRecord::Base
  has_many :memberships
  has_many :users, through: :memberships

  has_many :team_members, through: :memberships, source: :user, conditions: { 'memberships.role' => 'TeamMember' }

  has_one :ownership, class_name: 'Membership', conditions: { 'memberships.role' => "Owner" }
  has_one :owner, through: :ownership, source: :user
end


# Account.new.users.build
1.9.3 (main):0 > Account.new.users.build
+----+------+------+
| id | name | emai |
+----+------+------+
|    |      |      |
+----+------+------+
1 row in set


# Account.new.team_members.build
1.9.3 (main):0 > Account.new.team_members.build
+----+------+------+
| id | name | emai |
+----+------+------+
|    |      |      |
+----+------+------+
1 row in set


# Account.new.build_owner? owners.build? ...?
1.9.3 (main):0 > Account.new.build_owner
NoMethodError: undefined method `build_owner' for #<Account id: nil, name: nil, created_at: nil, updated_at: nil>


1.9.3 (main):0 > Account.new.owner.build
NoMethodError: undefined method `build' for nil:NilClass