Hi Philip,
Am I missing something here? (It is late after a long day, I admit!)
In the example case you've given, if the foreign key in Parts is set to
ON DELETE CASCADE, and you delete a row from Manufacturer, MySQL will
first delete the associated records in Parts before deleting the row
from Manufacturer - all you have to do is issue the DELETE FROM
Manufacturer WHERE .... query - MySQL will handle the rest.
If Parts was also the primary key table for another table (e.g.
PartNumbers) and that relationship was set to ON DELETE CASCADE, and you
issued the DELETE FROM Manufacturer... query, MySQL would delete the
associated records from all 3 tables automatically.
The only time you'd have to manually issue all 3 DELETE statements were
if your foreign keys didn't have the ON DELETE CASCADE option set.
Andy
Philip Pemberton wrote:
Hi,
First of all, I apologise in advance for any mind-altering, or
headache-inducing effects this question may have. I've spent the past
two days trying to figure it out, and all I've got to show for it is a
mostly-working recursive depth-first-search routine and an empty
packet of painkillers.
MySQL version: 5.0.67-0ubuntu6
I'm trying to write a code generator (in Python) that reads in a MySQL
database, enumerates all the tables, then produces INSERT, DELETE and
UPDATE code in PHP. The INSERT and UPDATE code generation was fairly
easy, and works quite well. What I'm having trouble with is the DELETE
code generator -- more specifically, resolving foreign key references.
Basically, what I have is a tree built in memory, so I can go:
tableinfo['thetable']['fieldname']['refs']
And get a complete list of all the tables (and the fields within that
table) that reference 'fieldname' in 'thetable'.
What I want is an answer to the question: "If all my foreign keys were
set to 'ON DELETE CASCADE', what would I need to do to delete row 'X'
in table 'Y' without violating any foreign key constraints?"
Here's an example. Let's say I've got these tables:
CREATE TABLE `Manufacturers` (
`idManufacturer` int(11) NOT NULL auto_increment,
`name` varchar(255) NOT NULL,
PRIMARY KEY (`idManufacturer`)
) ENGINE=InnoDB
CREATE TABLE `Parts` (
`idPart` int(11) NOT NULL auto_increment,
`idManufacturer` int(11) NOT NULL,
`partnumber` int(11) NOT NULL,
PRIMARY KEY (`idPart`),
KEY `Parts_idManufacturer_FKIndex` (`idManufacturer`),
CONSTRAINT `Parts_ibfk_1` FOREIGN KEY (`idManufacturer`) REFERENCES
`Manufacturers` (`idManufacturer`)
) ENGINE=InnoDB
And my database contains:
Manufacturers:
idManufacturer name
123 Any Company Inc.
Parts:
idPart idManufacturer partnumber
1 123 12345
Now, let's say I want to do this:
DELETE FROM Manufacturers WHERE idManufacturer=123
Because I have a part that references Manufacturer #123, I have to do
this instead:
DELETE FROM Parts WHERE idManufacturer=123
DELETE FROM Manufacturer WHERE idManufacturer=123
What I want is something I can feed the table definitions to, and the
name of the table I want to delete a row from (in this case
'Manufacturers'), and generate a list of the DELETE commands that
would allow me to delete that row while enforcing FK dependencies.
I figure this is going to have to work something like mathematical
expression evaluation -- build up a list of dependencies, then deal
with the deepest dependency first. Catch being I can't see an obvious
way to deal with generating the necessary DELETE commands without
having to write a massive "if recursion_level = 0 then
generate_a_straight_delete else if recursion_level = 1 then..."
statement...
Thanks,
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