Exploring let and let! in RSpec. Puzzled by how let and let! behaves, I decided to take https://www.relishapp.com/rspec/rspec-core/docs/helper-methods/let-and-let and modify it further to inspect let and let!

test_let_spec.rb

require 'spec_helper'

# comment out `config.order = "random"` in spec_helper or run rspec with `--order default` configuration (just for illustration)
# I have Database Cleaner set up in spec_helper too
$count1 = 0
$count2 = 0

describe "let and let!" do 

  describe "let!" do
    invocation_order = []

    let!(:count) do
      FactoryGirl.create(:user)
      invocation_order << :let!
      $count1 += 1
    end

    it "calls the helper method in a before hook" do
      invocation_order << :example
      invocation_order.should == [:let!, :example]
      $count1.should eq(1) # count was called before example implicitly
      count.should eq(1) # returns memoized version
      User.count.should == 1
    end

    it "recalls the helper method and executes the block before each example" do 
      invocation_order.should == [:let!, :example, :let!]
      $count1.should eq(2)
      count.should eq(2) # helper method only executed once before example
      invocation_order.should == [:let!, :example, :let!]
      User.count.should == 1 # database is teared down after each example
    end

  end

  describe "let" do
    let(:count) do 
      FactoryGirl.create(:user)
      $count2 += 1
    end

    it "memoizes the value within an example" do
      count.should eq(1)
      count.should eq(1) # helper method is only executed once per example
      User.count.should == 1
    end

    it "is not cached across examples" do
      count.should eq(2)
      count.should eq(2)
      User.count.should == 1
    end

  end
end