Question

I’m learning MVC and OOP, but something has me confused:

What is the difference between business and application logic?

Take a look at the following scenario of a payment and invoicing system:

Customers are invoiced, and when they pay the invoice it is acknowledged via a payments on the screen. The user enters the amount they have received from the customer into the system.

Now, is application logic the following (in the controller):

• User can not enter negative numbers
• User must enter input in the style of integers only or #.# or #.## or
  ##.## etc.

Business logic (in the model):

• The amount entered can not exceed the amount on the invoice (ie
  what’s due)
• And/or the amount entered can not exceed the amount owed if part
  payments have been made previously

Do I have the right idea?

Answer 1

Normally you should put all business logic, including that for formatting and validation into the model. So not entering a negative number is a focus for the model not the controller. Read up on Skinny Controller, Fat Model and maybe the presenter pattern using Draper. Ryan Bates has covered a fair amount of this stuff recently on Railscasts. Probably good to look into that too.