Rspec let variable in before block not changing between examples - ruby-on-rails

Question

In rspec 3.2, I have something based on the following pseudocode:
context 'my test context' do
before do
method_that_uses(error_message)
end
subject { post :my_action, params: a_bunch_of_params }
let(:error_message) { 'error' }
it { is_expected.to raise_error(MyException) }
let(:error_message) { 'different error' }
it { is_expected.to redirect_to(a_path) }
let(:error_message) { 'third error' }
it { is_expected.to redirect_to(another_path) }
end
Every example runs with error_message set to third error. I confirmed this by running pry from the before hook as well. How can I get the desired behavior?

Answer 1

That happens because internally, let uses define_method as seen in the source for let. You can create a quick example
class A
def create_method(name, &block)
self.class.send(:define_method, name, &block)
end
end
a = A.new
a.create_method(:foo) { puts "bar" }
a.create_method(:foo) { puts "baz" }
a.foo
and run that and you would see that define_method overrides the previous method with the new one. So in your example, you are creating a method and then overriding it's definition twice before you ever get a chance to call it.
You are wanting to run each error_message in it's own context like this:
def method_that_uses(e)
puts "running with: #{e}"
end
context 'my test context' do
before do
method_that_uses(error_message)
end
context 'error' do
let(:error_message) { 'error' }
it { puts "run one" }
end
context 'second error' do
let(:error_message) { 'different error' }
it { puts "run two" }
end
context' third error' do
let(:error_message) { 'third error' }
it { puts "run three" }
end
end
which, when run, outputs
running with: error
run one
.running with: different error
run two
.running with: third error
run three
.
This works because describe and context blocks create a new ExampleGroup (source) and ExampleGroup states
Example group bodies (e.g. describe or context blocks) are evaluated in the context of a new subclass of ExampleGroup.
So, let(:error_message) is now defining those methods on different subclasses.