Re: Mixing OO and DB
Date: Sun, 2 Mar 2008 13:01:20 -0600
On 2008-02-28 12:03:06 -0600, frebe <frebe73_at_gmail.com> said:
>> So, for instance, if we must fetch an employee record frequently in >> various parts of our application it is better to use a function like >> this: >> >> Employee e = Employee.find("Bob"): >> >> Than it is to use raw SQL like this all through our app. >> >> DataRow e = exec("Select * from EmployeeTable where name = 'Bob';"
> Why is the first line better than the last line?
It's smaller, more communicative, and can be polymorphically deployed.
>> Of course we can implement other kinds of 'find' methods. >> find_by_date, find_by_salary, etc, etc. Every different query we use >> on a regular basis, we can create a verb for in our DSL. That verb is >> a function call of some kind.
> And for queries we don't use on a regular basis? We can use "raw" SQL,
Write methods for them.
>> So we have not lost the power of the flexible query language.
> If you still allow "raw" SQL for queries not used on a regular basis,
> the power is not lost. But if you must have a find_by_xxx, for every
> possible select statement, the power is lost.
Not at all. All we are doing is giving each query a name. That name is the name of the method that is implemented by the SQL. This allows the programmer to commnicate the *intent* of the query, rather than it's implementation.
e.g. Employee.find_all_eligible_for_retirement() is a lot better than a
long and complex SQL statement.
> What is the benefit with hiding SQL from the bulk of the application?
>> That allows us to create a polymorphic API below which we can swap out >> different database implementations, or even stubs and dummies .
> If you have a lot of find_by_xxx methods, swapping would still be
> quite difficult.
Swapping is done on a test by test basis. You don't have to swap everyting every time. For example, you want to test the functions that render all eligible employees in a scrolling list on a screen. You replace the Employee.find_all_eligible_employees method with an implementation that simply returns a canned list.
-- Robert C. Martin (Uncle Bob) | email: unclebob_at_objectmentor.com Object Mentor Inc. | blog: www.butunclebob.com The Agile Transition Experts | web: www.objectmentor.com 800-338-6716 |Received on Sun Mar 02 2008 - 20:01:20 CET