example_fixed.rb

describe Something do
  subject{ Something.new }
  let(:my_object){ Namespace::MyClass.new }

  before do
    allow(my_object).to receive(:foo)
    allow(subject).to receive(:my_object).and_return(my_object)
    subject.do_something
  end

  specify { expect(my_object).to have_received(:foo).once }
end

class Something
  def do_something(class_name)
    my_object.foo
  end

  def my_object(class_name)
    "Namespace::#{class_name}".constantize.new
  end
end