We use thoughtbot’s FactoryGirl extensively in our daily spec (& feature) writing. One recent problem we encounter is the failure to correctly set the associations between the following 2 example models.

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class Blog < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :blog
end

The factory definitions are as follow:

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FactoryGirl.define do
  factory :comment do
    # (blah blah)
    blog
  end
end

FactoryGirl.define do
  factory :blog do
    # (blah blah)

    factory :commented_blog do
      comments { [association(:comment)] }
    end
  end
end

The problem with the above definitions is that calling Factory(:commented_blog) creates 2 blogs & 1 comment instead of the expected 1 blog & 1 comment, and the comment isn’t owned by the intended blog.

Fixing this problem turns up to be pretty simple, all we need to do is when creating a comment, we enforces the desired ownership:

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FactoryGirl.define do
  factory :blog do
    # (blah blah)

    factory :commented_blog do
      # Explicte declaring the rightful owner of the comment
      after_create {|blog| Factory(:comment, blog: blog) }
    end
  end
end

If you have some time to spare, u really should take a good read of this doc