Re: Design Question
From: Robert C. Martin (unclebob_at_objectmentor.com)
Date: 04/24/04
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Date: Sat, 24 Apr 2004 08:20:29 -0500
On 22 Apr 2004 19:45:03 -0700, nunaya2004@yahoo.com (Nunaya) wrote:
>We have developed two objects called "customer" and "salesrep". For
>simplicity, assume that these two objects will talk directly to a
>database. The database has three tables: customers, customersalesreps
>(allows multiple salesreps to be associated with a customer), and
>salesreps. The customer object has business rules that allow
>manipulation of customer information (add,update,delete,select,etc).
>The salesrep object has business rules that allow manipulation of
>salesrep information (add,update,delete,select,etc).
>
>Question 1: Where is the proper place to put a method called
>"AddCustomerSalesrep" that adds a new salesrep to the
>customersalesreps table with the customerid and salesrepid? Is it in
>the customer or salesrep object? (Or is it in a third object we
>should create called "customersalesrep"?)
>
>Question 2: Assume one of the business rules desired is to validate
>the salesrepid prior to adding a new salesrep to a customer. Also
>assume that the salesrep object already has a method called
>"issalesrepvalid" which takes a salesrepid and makes sure it exists in
>the salesreps table. If we put AddCustomerSalesrep in the customer
>object, would we want to validate the salesrepid by having the
>customer object instantiate the salesrep object and call the method
>called "issalesrepvalid"? Does this break the principle of
>encapsulation or is there a better way to design this?
>
>Thanks in advance for any insight.
You are allowing the structure of the database to pollute your
thinking. The structure of the database (i.e. the tables) are details
that you should ignore for the time being. Instead, build your
Customer and SelseRep classes without any knowledge of the database at
all. Don't even admit that there *is* a database. Get your
application working in RAM first, then add the database as your last
step.
This will force you to focus on business rules. It will cause the
structure of your classes and objects to support the business rules
first. Then, later, as you need persistence, you can shim the
database behind the business rules in a way that does not warp the
object model.
-----
Robert C. Martin (Uncle Bob)
Object Mentor Inc.
unclebob @ objectmentor . com
800-338-6716
"Distinguishing between the author
and the writing is the essence of civilized debate."
-- Daniel Parker
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