Question

How do you mock a push into a class array variable using Rspec? Here is an over-simplified example:

class Foo
  attr_accessor :bar
  def initialize
    @bar = []
  end
end

def some_method(foo)
  foo.bar << "a"
end

Say I want to write a spec for some_method that “it should push a new value to bar”. How do I do that?

foo = Foo.new
foo.should_receive(WHAT GOES HERE???).with("a")
some_method(foo)

Answer 1

Why mock anything? You only need to mock something when it is a dependency that you are trying to isolate from what you are actually trying to test. In your case, you seem to be trying to verify that calling some_method results in adding an item to a property of an object you passed in. You can simply test it directly:

foo = Foo.new
some_method(foo)
foo.bar.should == ["a"]

foo2 = Foo.new
foo2.bar = ["z", "q"]
some_method(foo2)
foo2.bar.should == ["z", "q", "a"]

# TODO: Cover situation when foo.bar is nil since it is available as attr_accessor 
# and can be set from outside of the instance

* EDITED After Comments Below **

foo = Foo.new
bar = mock
foo.should_receive(:bar).and_return bar
bar.should_receive(:<<).with("a")
some_method(foo)