|
|
Posted by Dikkie Dik on 11/19/66 11:47
>> Quite simple. If the primary key in your application is numeric, you got
>> it from the database. Otherwise, how would you know the key value?
>
> Because you know it?
>
> For example, if I'm keeping a table of my customers' credit cards, I might
> use:
>
> customer_id (int, foreign key)
> card_number (big int, primary key)
> card_holder_name (varchar)
> expiry_date (timestamp)
>
> If I'm inserting a new record into the table then I already know the
> primary key, don't I?
>
No, you don't. You know a FIELD value of your record, NOT its primary
key. That you wish to combine them (that is always going to get you in
trouble sooner or later) does not change anything. For clarity: A
primary key is only a unique pointer to a record and nothing more.
Especially, primary key values should never be related to the data in
the record.
Let me show it. Suppose this card object is a new one. In this case, you
do NOT know its primary key. You can't, because the primary key value
does not exist yet. Oh, you might know what it WILL be, but I could also
know what it would be if I used an autonumber. The card number column in
you table serves to purposes: it holds both the identity of the record
and some data.
You don't believe me? Just lookup your "known" record in the database.
You won't find it. It does not exist. You will only find it if you know
its primary key value. And for the record to have a primary key value,
it must exist in the database first.
But apart from that, your code does know whether the record comes from
the database or from another source. If you can't trust your primary
keys, you could have the table wrapper just set a boolean when getting
the record from the database. The original question was: how do you know
that a record comes from the database? If you want to know that, your
database code has to be held responsible for keeping track of that. If
you can't do that with the primary key because it interferes with data,
just find another way. Anyhow, your table wrapper knows if it came from
the storage.
Navigation:
[Reply to this message]
|