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RSpec: before and let

11 Oct 2013

Understanding when/why to use before(:all) vs before(:each) vs let vs let!

I’ve learned to use before and let to DRY up RSpec tests, as well as make them more readable. However, I didn’t have a complete grasp of what each of them did. I decided to write some code that helped me understand exactly what each of them do, and when I should be using each one.

before(:all)

before(:all)
  FactoryGirl.create(:tag, :permalink => "before-all")
end
  • runs before each describe/context block
  • each insert persists in the database (does not get automatically cleaned up/deleted)

before-all code example

After the above code example is run, there will be 3 records stored in the Tag table.

before(:each)

before(:each)
  FactoryGirl.create(:tag, :permalink => "before-each")
end
  • runs before each example (it-block)
  • cleans itself up after each example
  • slower than before(:all) if you have a lot of FactoryGirl database inserts

before-each code example

let

let(:tag) { FactoryGirl.create(:tag, :permalink => "let") }
  • lazy-loads the object
  • cached across multiple calls in the same example but not across examples
  • cleans itself up after it-block

let code example

let-bang

let(:tag2) { FactoryGirl.create(:tag, :permalink => "let-bang") }
  • runs right away at the beginning of each it-block
  • cached across multiple calls in the same example but not across examples
  • cleans itself up after it-block

let-bang code example

Tips

  • Most of the time you should be using let and/or before(:each)
  • let is almost always preferable to reference objects
    • instance variables spring into existence only when necessary
    • before(:each) will create all objects even if the example doesn’t use it
  • before(:all) shouldn’t really be used for variable setup, because it introduces an order dependency between specs
    • Good use cases: opening a network connection, pre-seeding caches
  • If you use before(:all), think about using corresponding after(:all) for clean-up
  • before(:each) can be used to instantiate objects to make specs easier to read, if there is a lot of setup involved before an example
  • If you need records in the db without first referencing an object, before(:each) seems to be a better choice than let!
  • Don’t mix before and let together, i.e.
 before(:each); let(:foo) {}; end 
* You'll get a warning in RSpec 2 but not RSpec 1: `let` and `subject` declarations are not intended to be called in a `before(:all)` hook, as they exist to define state that is reset between each example, while `before(:all)` exists to define state that is shared across examples in an example group.

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