Pablo Fernandez

Rails come with some awesome assertion methods for writing tests:

assert_difference("User.count", +1) do
  create_a_user
end

That asserts that the count of user was incremented by one. The plus sign is not needed, that’s just an integer, I add it to make things clear. You can mix several of this expressions into one assert_difference:

assert_difference(["User.count", "Profile.count"], +1) do
  create_a_user
end

That works as expected, it asserts that both users and profiles were incremented by one. The problem I have is that I often found myself doing this:

assert_difference "User.count", +1 do
  assert_difference "Admin.count", 0 do
    assert_difference "Message.count", +3 do  # We send three welcome messages to each user, like Gmail.
      create_a_user
    end
  end
end

That looks ugly. Let’s try something different:

assert_difference("User.count" => +1, "Admin.count" => 0, "Message.count" => +3) do
  create_a_user
end

Well, that looks nicer, and straightforward, so I implemented it (starting from Rails 3 assert_difference):

def assert_difference(expressions, difference = 1, message = nil, &block)
  b = block.send(:binding)
  if !expressions.is_a? Hash
    exps = Array.wrap(expressions)
    expressions = {}
    exps.each { |e| expressions[e] = difference }
  end

  before = {}
  expressions.each {|exp, _| before[exp] = eval(exp, b)}

  yield

  expressions.each do |exp, diff|
    error = "#{exp.inspect} didn't change by #{diff}"
    error = "#{message}.\n#{error}" if message
    assert_equal(before[exp] + diff, eval(exp, b), error)
  end
end

Do you like it? If you do, let me know and I might turn this into a patch for Rails 3 (and then let them now, otherwise they’ll ignore it).

Update: this is now a gem.