Rails / RSpec: How to test #initialize method?

For me, expectations are about designing conversations among collaborators. So, you have to decide - is #pick_seed an internal implementation detail or part of a collaborating role's interface?

If pick_seed is an implementation detail, an expectation is the wrong tool for the job. And, since you have an accessor for seed, you can proceed thusly (notice the one-assertion-per-example):

class Generator  attr_accessor :seed  def initialize(seed = nil)    @seed = seed || pick_seed  end  def pick_seed    Time.now.to_i  endenddescribe Generator do  context "creating" do     context "when a seed is specified" do      it "uses that seed" do        seed = 123        generator = Generator.new(seed)        generator.seed.should == seed      end    end    context "when a seed is not specified" do      it "creates its own seed" do        generator = Generator.new        generator.seed.should_not be_nil      end    end  endend

OTOH, if picking the seed is part of the "seed picker" role, then mocks are valuable in designing the seed picker, and dependency injection is a standard method of assigning the roles. You could write something like:

class GeneratorWithCollaborator  attr_accessor :seed  def initialize(seed = nil, seed_picker = self)    @seed = seed || seed_picker.pick_seed  end  def pick_seed    Time.now.to_i  endenddescribe GeneratorWithCollaborator do  context "creating" do    context "when a seed is specified" do      it "uses that seed" do        seed = 123        seed_picker = double('seed picker')        seed_picker.should_not_receive(:pick_seed)        generator = GeneratorWithCollaborator.new(seed, seed_picker)        generator.pick_seed      end    end    context "when a seed is not specified" do      it "delegates to its seed picker" do        seed_picker = double('seed picker')        seed_picker.should_receive(:pick_seed)        generator = GeneratorWithCollaborator.new(nil, seed_picker)      end    end  endend