Stubbing constants that are used as a side effect of a let! function

rails_helper.rb

# Rails helper config stuff...

# Because `let!' is always run before any `before` calls, you have to manually
# overide the constant in the rails helper
SomeClass.send :remove_const, 'FILE_NAME'
SomeClass.const_set 'FILE_NAME', 'spec/fixtures/files/something/cool.yml'

some_class.rb

class SomeClass
  FILE_NAME = 'something/cool.yml'
  
  attr_reader :file
  
  def load_file
    @file = YAML.load_file(FILE_NAME)
  end
end

some_spec.rb

require 'rails_helper'

RSpec.describe SomeClass do
  let(:some_class) { SomeClass.new }
  let!(:load_file) { some_class.load_file }
  
  it 'should now use the stubbed constant' do
    expect(some_class.file).
      to eq YAML.load_file('spec/fixtures/files/something/cool.yml')
  end
end