Question

For example, in FactoryGirl, it allows you to do something like

factory :user, :class => User do
  sequence :username do |n|
    "blahblah#{n}"
  end
end

I’ve looked through the source code at https://github.com/thoughtbot/factory_girl but can’t seem to figure out how they are achieving this (it loks like it’s some type of proxy?). That is, how is it we are able to access the sequence method?

Answer 1

They are using instance_eval.

This means that inside the block self (and thus the object sequence is called on) isn’t what it would otherwise be (probably the top level object in this case) but the object that instance_eval was called on (in this case an object that implements the constructs of the factory girl syntax.

It’s often used with DSLs (another example is the routes.rb file in rails >= 3) because without it you’d have to do something like

factory ... do |f|
  f.sequence ...
end

which tend to distract from the DSL.

One tiny example illustrating its effects

foo = "123456"
foo.instance_eval do
  length
end
#=> 6

With a ‘normal’ block this would have raised an error, because the top level object has no length method, but because instance_eval is used length is called on the string instead.