module ActiveRecord::Persistence

def increment(attribute, by = 1)

Only makes sense for number-based attributes. Returns +self+.
The increment is performed directly on the underlying attribute, no setter is invoked.
Initializes +attribute+ to zero if +nil+ and adds the value passed as +by+ (default is 1). 
def increment(attribute, by = 1)
  self[attribute] ||= 0
  self[attribute] += by
  self
end