account.rb

class Account < ActiveRecord::Base
  
  has_many :users
  has_many :account_admin_permissions
  has_many :admins, :through => :account_admin_permissions, :class_name => 'User'
  
end

account_admin_permission.rb

class AccountAdminPermission < ActiveRecord::Base
  
  belongs_to :account
  belongs_to :user
  
end

factories.rb

Factory.define :account do |a|
  a.name { Factory.next(:name) }
end

Factory.define :user do |u|
  u.name { "%s User" % Factory.next(:name) }
  u.login { |u| u.name.downcase.gsub(/\s/, '.') }
  u.email { |u| "%s@example.com" % u.login }
  u.password "secret"
  u.password_confirmation { |u| u.password }
  u.account { |u| u.association(:account) }
  
  # TRIED THIS:
  u.account_admin_permission { |u| u.association(:account_admin_permission, :account => u.account) }
end

Factory.define :account_admin_permission do |p|
  p.association(:user)
  p.association(:account)
  # BOTH VERSIONS result in infinite loops
  # p.user { |u| u.association(:user) }
  # p.account { |u| u.association(:account) }
end

user.rb

class User < ActiveRecord::Base
  
  belongs_to :account
  has_one :account_admin_permission
  
end