PHP update MYSQL many-to-many relation - php

Question

I have a many-to-many relation, implemented with an association table in MySQL. I have a table for children and a table for parents. A child can have multiple parents, saved in the parent_child_link association table with their ID's.
Children can be updated through a HTML form, the parents are in a HTML multi-select. Now I need to update the record in the database, but my solution is not very efficient. Here's in pseudocode what I do:
Update the child information where child_id=x
Delete all the current associations in parent_child_link where child_id=x
Insert the new associations
This solution works great, but when the the parents weren't changed, e.g. only the name of the child was changed, then there are 2 unnecessary queries executed. How can I avoid those unnecessary queries? Is there some way to check if the parents in the multi-select didn't change?
Ofcourse I could just ignore all this hassle, because it already works, but I really like to keep things as efficient as possible.

Answer 1

I have the same question and figured out my solution as I was reading.
When I am ready to process the submitted entries, I first do a query to get the current associations and call that array $original_list. The submitted list I will call $submitted_list.
$original_list = array(3,5,7);
$submitted_list = array(1,2,3);
Then I just need to figure out 1) which items to delete (no longer exist) and 2) which items to add (new associations). Items in both lists do not get touched.
$delete_list = array_diff($original_list, $submitted_list);
$insert_list = array_diff($submitted_list, $original_list);
foreach($delete_list as $item) {
// delete $item from DB
}
foreach($insert_list as $item) {
// insert item in db
}
Would love to know if others feel this a valid solution.

Answer 2

Try solving it in the database, not in the application layer by using ON UPDATE CASCADE and ON DELETE CASCADE in the definition of the child table.
A slightly revised example form the MySQL site:
CREATE TABLE parent (id INT NOT NULL,
PRIMARY KEY (id)
) ENGINE=INNODB;
CREATE TABLE child (id INT, parent_id INT,
INDEX par_ind (parent_id),
FOREIGN KEY (parent_id) REFERENCES parent(id)
ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=INNODB;
Check out the docs here: http://dev.mysql.com/doc/refman/5.5/en/innodb-foreign-key-constraints.html
EDIT: For your many-to-many relation you can use something like:
CREATE TABLE parent_child_link (
parent_id INT NOT NULL,
child_id INT NOT NULL,
PRIMARY KEY(parent_id, child_id),
FOREIGN KEY (parent_id) REFERENCES parent(id)
ON DELETE CASCADE ON UPDATE CASCADE,
FOREIGN KEY (child_id) REFERENCES child(id)
ON DELETE CASCADE ON UPDATE CASCADE
);
Hope this helps.

Answer 3

Your solution is fine.
In your case you could "optimize" the process by making a query to retrieve the parents and check with the multi-select data if any changes has occurred.
Then you only perform the two delete and insert queries if needed. The counterpart is that when you actually changed the parents, then there will be 3 queries instead of 2.
So you should ask you if the you are about to modify the parents very often. In this case you should stick to your original solution to avoid an extra select query.
If you think the parents won't be updated very often, then you can go with the above solution. When you update the child info only, only one query is performed. When you also update the parents, 3 queries are performed.
When you go with the second solution, the delete and insert queries can be optimized too to only perform what is required (only delete the parents that are not his parents anymore and only insert new parent links).
PHP array functions can be helpful for that.

Answer 4

If you want to keep you'r current way of doing it, but just optimizing, you could wrap the queries in IF statements.
Like:
if ( isset ( $parent_name_change )){
// run query
}