module ThoughtBot::Shoulda::Macros

def should_change(expression, options = {})

should_change "@post.title", :to => "new" # => assert the value changed from anything other than "new"
should_change "@post.title", :from => "old" # => assert the value changed to anything other than "old"
should_change "@post.title" # => assert the value changed in some way

Combinations of :by, :from, and :to are allowed:

should_change "@post.title", :from => "old", :to => "new"
should_change "Post.count", :from => 0, :to => 1

may also specify :from and :to options:
difference between the before and after values of the expression. You
As shown in this example, the :by option expects a numeric

end
should_change "Post.count", :by => 1
setup { Post.create }
context "Creating a post"

Example:

should_not_change.
assertion, but supports more than just numeric values. See also
is run. This is similar to Active Support's assert_difference
of an expression that is run before and after the current setup block
Macro that creates a test asserting a change between the return value 
def should_change(expression, options = {})
  by, from, to = get_options!([options], :by, :from, :to)
  stmt = "change #{expression.inspect}"
  stmt << " from #{from.inspect}" if from
  stmt << " to #{to.inspect}" if to
  stmt << " by #{by.inspect}" if by
  expression_eval = lambda { eval(expression) }
  before = lambda { @_before_should_change = expression_eval.bind(self).call }
  should stmt, :before => before do
    old_value = @_before_should_change
    new_value = expression_eval.bind(self).call
    assert_operator from, :===, old_value, "#{expression.inspect} did not originally match #{from.inspect}" if from
    assert_not_equal old_value, new_value, "#{expression.inspect} did not change" unless by == 0
    assert_operator to, :===, new_value, "#{expression.inspect} was not changed to match #{to.inspect}" if to
    assert_equal old_value + by, new_value if by
  end
end

def should_not_change(expression)

end
should_not_change "Post.count"
setup { @post.update_attributes(:title => "new") }
context "Updating a post"

Example:

is run. This is the logical opposite of should_change.
of an expression that is run before and after the current setup block
Macro that creates a test asserting no change between the return value 
def should_not_change(expression)
  expression_eval = lambda { eval(expression) }
  before = lambda { @_before_should_not_change = expression_eval.bind(self).call }
  should "not change #{expression.inspect}", :before => before do
    new_value = expression_eval.bind(self).call
    assert_equal @_before_should_not_change, new_value, "#{expression.inspect} changed"
  end
end