FactoryGirl associations without callbacks

Given a posts.rb factory:

factory :post do
  user
  text "hello"
end

Following the guide you would do this:

factory :user do
  name "bob"

  transient do
    post_count 1
  end

  after(:create) do |user, evaluator|
    create_list(:post, post_count, user: user, text: "#{user.name} is cool")
  end
end

This is alright, but leads to issues. For example, if you merely build a user, then that user will not have any posts.

>> FactoryGirl.build(:user).posts.size
=> 0
>> FactoryGirl.create(:user).posts.size
=> 2

Also, in a more complex association structure it can be hard to access posts.

factory :oeuvre do
  after(:create) do |oeuvre, evaluator|
    user = build :user
    posts = create_list(:posts, 2, user: user)
    replies = create_list(:replies, 2, user: user)
    oeuvre.works.push *posts
    oeuvre.works.push *replies
  end
end

A better way

An under-documented fact about attribute blocks is that self is the evaluator object from the after create block. This allows you to call any transient, attribute, or association on the factory.

factory :user do
  name "bob"
  posts { build_list(:post, post_count, user: nil, text: "#{name} is cool") }
  replies { build_list(:reply, reply_count, user: nil, text: "#{name} is cool") }

  transient do
    post_count 1
    reply_count 1
  end
end

factory :oeuvre do
  user { build :user }
  works { user.posts + user.replies }
end

As you can see above, the oeuvre factory is now much simpler. In addition
build now gives the correct sizes.

>> FactoryGirl.build(:user).posts.size
=> 2
>> FactoryGirl.create(:user).posts.size
=> 2

Last but not least, creating objects with association blocks as opposed to foo
after create blocks is significantly faster

factory :user_a, class: 'User' do
  after(:create) {|u,e| create_list(:post, 2, user: u) }
end

factory :user_b do
  posts { build_list(:post, 2, user: nil) }
end

factory :post do
  user
end

Benchmark.ips {|r|
  r.report("after create") { FactoryGirl.create :user_a }
  r.report("assn create") { FactoryGirl.create :user_b }
  r.report("assn build") { FactoryGirl.build :user_b }
}

Warming up --------------------------------------
        after create     3.000  i/100ms
         assn create     8.000  i/100ms
          assn build   207.000  i/100ms
Calculating -------------------------------------
        after create     35.724  (±22.4%) i/s -    171.000  in   5.040311s
         assn create     91.368  (±17.5%) i/s -    448.000  in   5.049313s
          assn build      2.061k (± 1.7%) i/s -     10.350k in   5.023673s