Question

class Invoice
  def Invoice.generate(order_id, charge_amount, credited_amount = 0.0)
    Invoice.new(:order_id => order_id, :amount => charge_amount, :invoice_type => PURCHASE, :credited_amount => credited_amount)
  end
end

Why would you create Invoice.generate inside Invoice class rather than self.generate?

Answer 1

self.generate is easier to work with, whereas Invoice.generate is arguably more explicit. Other than that, there’s no difference between the two.

Explanation

You can define a method on any instance using this form

def receiver.method(args) 
end

Check this out

class Foo
end

def Foo.bar
  "bar"
end

Foo.bar # => "bar"

And yes, I mean any instance. It’s absolutely possible that one instance has some method while another doesn’t

f = Foo.new

def f.quux
  'quux'
end

f2 = Foo.new

f.quux # => "quux"
f2.quux # => # ~> -:20:in `<main>': undefined method `quux' for #<Foo:0x007fe4e904a6c0> (NoMethodError)

A reminder: inside of class definition (but outside of method definitions) self points to that class.

class Foo
  # self is Foo
end

So, armed with this knowledge, the difference between self.generate and Invoice.generate should be obvious.