Super handy way to modify and return the instance of an object

post_spec.rb

# Ruby 1.9
# Object.tap
# Here we use tap to set the post factory's author (since Factory Girl doesn't create singleton instances).
# Using tap passes a block where you can "tap" into the object.
# At the end of the block, the object is returned.

let(:author) { Factory(:author) }
let(:post) {
  Factory(:post).tap do |p|
    post.update_attribute(:author, author)
  end
}

# Now we can write tests against post.author like this:

it "should have the right author" do
  post.author.should == author
end