title: Associated object does not have a primary key
body: <p>I still can't find a solution to fix an association factory issue when using Sequel.</p>
  
  <p>I have two models relying on <code>one_to_many</code>, which is the same as <code>has_many</code>in Active Record, and <code>many_to_one</code>, which is the same as <code>belongs_to</code> in Active Record.</p>
  
  <p>Here are the defined factories:</p>
  
  <pre><code>FactoryGirl.define do
    to_create { |instance| instance.save }
    factory :post do
      title  "some title"
    end
  end
  
  FactoryGirl.define do
    to_create { |instance| instance.save }
    factory :comment do
      content    "some content"
      association :post, strategy: :build
    end
  end
  </code></pre>
  
  <p>When running <code>build(:comment)</code>, it fails with:</p>
  
  <pre><code>Associated object does not have a primary key. 
  </code></pre>
  
  <p>Does anyone have an idea how to fix that? I can always build/create a post first, then sign it to a comment, but it is tedious. More than that, I'll have to remove <code>association :post, strategy: :build</code> and use some Integer random value. </p>
  
  <p>I'm using: </p>
  
  <ul>
  <li><code>factory_girl_rails 4.8.0</code></li>
  <li><code>ruby 2.4.0</code></li>
  <li><code>sequel-rails 0.9.15</code></li>
  <li><code>sequel 4.45.0</code></li>
  </ul>