factory_girl_race_condition.rb

class Merchant < ActiveRecord::Base
  def foobarable?
    # ...
  end
end

class Customer < ActiveRecord::Base
  belongs_to :merchant
  def foobar=(something_else)
    self[:foobar] = something_else if merchant.foobarable?
  end
end

FactoryGirl.define do
  factory :merchant do
  end
  factory :customer do
    association :merchant
  end
end

# How can I make sure that the merchant attribute/association is set first so 
# that the foobar attribute checks my passed in merchant?
c = FactoryGirl.create :customer, :merchant => @some_merchant, :foobar => 'xyz'

Do two lines work?

c = FactoryGirl.create :customer, :merchant => @some_merchant
c.foobar = 'xyz'

Yes, but that should go without saying too. FYI, it would have to be broken up in a build then manual save. So something like this.

c = FactoryGirl.create(:customer, :merchant => @some_merchant).tap { |c| c.foobar = 'xyz' ; c.save! }

FWIW, here is the reason and talk around why.
thoughtbot/factory_bot#140

Looks to me your trouble with using Factory.create here shows that the corresponding Customer.create would also be unreliable (order matters, but Hash has no order).

Exactly, it does in 1.9 and that is also mentioned in the issue link.

I don't know if "that's a cure". Looks like asking for trouble: if someone isn't aware of the ordering requirement.

It seems to me when a Hash is accepted as an argument, then the message is that order is not important (in other languages too. of course).

I'm not disagreeing on that too, hence why I am using the patch till I hear other feedback from the authors.