How do you call into a Table Class

[Bastiaan Olij]

Hey Das,

I could write an entire book on this subject :)

The short answer is, what you suggest is a bad idea. This will probably
be the longest part of your learning curve wrapping your head around
object orientation and why it works. Until you grasp it, it'll feel like
its constantly throwing up needless roadblocks. Once you grasp it you'll
find you'll be writing much more stable and usable code.

There is a golden rule in OO and that is that an object should never
know about its outside world. If you create a class (in your example a
table class) and the execution of a method of that class depends on the
existence of something outside of that class (in your example variables
on a window), then suddenly that object has a number of problems, I'll
list two of them, but its the tip of the iceberg.

Think of the fact that unlike Omnis Classic, Omnis Studio allows you to
have multiple instances of a class. For instance, in classic your table
is represented by a file class, a file class can only hold one record at
a time. A function you wrote only had to deal with one such record, you
could cut lots of corners knowing this.
Now in Studio I have a table class, instantiated as many times as I
want. Lets say I have a window that enters a customer and for that
customer I want to record two addresses. A mailing address and a
delivery address. Good database design tells me I'll have a table with
customers, and I will have a table with addresses. Each customer will
have two records in the address table.
On my window I want to show those two addresses so I have two instance
variables of type row both defined from my address table class.
Now my function on my table class uses a variable on my window... But I
have two records... I need two different variables, but I hard coded it
and I told the table class it should use a variable on my window called X.

For the second example, think of having a table class for an invoice
table. Again on my window I have a row variable defined from this table
class. For whatever reason this table class assumes certain other
variables exist on my window and uses those variables in its methods.
All good and dandy so far. My window works, I'm happy.
Unfortunately for my colleague however, some months later she is tasked
with writing a process that creates invoices for all un-invoice work for
all our clients. Our client is tired of having to do those manually
through the window I created so they want to automate this. She creates
a new window that gathers all the un-invoiced work, loops through this
data and for every entry creates an invoice that invoices that work.
Inside this look she creates a row variable, she defines it from my
invoice table class, populates the columns and calls $insert.
$insert errors out because $insert relies on a bunch of things that only
exist on my window that was written for a user to enter a single invoice.

Now the thing is, there is one concept that is hard to get when moving
from traditional top down programming to object oriented programming.
You are used to variables with data existing somewhere, and handing
those variables over to a function.
In OO methods work on data. Object classes remain the easiest ones to
explain this with. You have an object, that object encapsulates a number
of variables, and it has a number of methods that perform actions on and
with that data. The object should be self contained, it encapsulates the
variables and logic. In practice, once used properly, it means you're
variables won't exist on your window, they will exist within the object,
most of the time and yes, those few times you need to use a variable in
your window, you'll have to pass that as a parameter to a method.

A table class simply adds to that columns of your table but
functionality they are no different than using an object which has an
instance variable for each column in your table. Practically table
classes just make working with those columns easier because it holds
more meta data about your table that allows you to generate the required
SQL to interact with your database.

Anyways, enough information overload :)

Cheers,

Bas


On 30/1/18 11:42 am, Das Goravani wrote:
> Mayada, 
>
> Why do you prefer the object class, and, the call is not the same because a table is INSIDE a row or list, the object is not, as far I understand, and, what about losing touch with your variables inside the window you’re calling from?  I can’t lose touch with them… I have yet to see how a table works out, I’m just trying it now… tables and objects… do they keep scope with variables in windows, declared in windows, or do you have to pass everything as parameters or references, that I am curious about.  Thank you for your valuable time. 
>
> Das Goravani
> We both have nice names