[Lazarus] [OT] How to delete a record/object being used
Michael Van Canneyt
michael at freepascal.org
Tue Sep 9 15:19:50 CEST 2008
On Tue, 9 Sep 2008, Graeme Geldenhuys wrote:
> [...this is probably off-topic as it's related to database
> applications, not Lazarus specifically...]
> How do you guys handle such a case in database applications. Say you
> have an TAddressType object/record. Now you have a lot of Contacts
> with address information stored in a database (and address book type
> Now I want to delete a specific AddressType, but that AddressType is
> being used by some Contacts in there address information.
> What's my choices? How do you handle such a case?
> * What happens if you simply try and delete the AddressType?
> * DB referential integrity might stop the delete, but gives a lovely
> error. If I don't have referential integrity enabled, I have ghost
> address types lying around.
Not if all foreign keys to the table have the cascade delete option specified,
then it is automatic. This is the option I most of the time use.
So something like
Create table AdresType (
ID INT PRIMARY KEY,
CREATE TABLE ADDRESSES(
ALTER table ADDRESSES ADD CONSTRAINT R_ADDRESSES_AdresType FOREIGN KEY
(AddressType) REFERENCES AdresType(ID) ON DELETE CASCADE;
When you delete AdresType record with ID 1, all ADDRESSES records that have
AddressType =1 will automatically be deleted.
> * Do I have to create some search query to run through all tables looking
> for that AddressType?
That is the manual way. I use it if a foreign key is not an option.
> * Must I maybe extend my AddressType table to include some 'Active'
> boolean field. If set to False, it simply hides that record from the
> GUI, but still available in the database?
Some people do this, but I recommend against it, as you end up with a lot
of junk in your database if it is actively used.
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