FactoryGirlのattributes_forがassociationを生成してくれない ref: http://qiita.com/blueplanet/items/82a349f99d0faa925a85

file0.rb

# model
class Group
  has_many :users
end

class User
  belongs_to :group

  validates :group_id, presence: true
end

# factory
FactoryGirl.define do
  factory :user do
    sequence(:name) {|n| "user #{n}" } 
    group
  end
end

file1.rb

user_params = FactoryGirl.attributes_for(:user)

file2.rb

user_params = FactoryGirl.build(:user).attributes

file3.rb

user = FactoryGirl.build(:user)
user_params = user.attributes.merge(password: user.password, password_confirmation: user.password_confirmation)