I'm having trouble with calculating a derived value using a combination of triggers and foreign key constraints.
There are 3 tables involved in this arrangement
responses ------------------------ id_responses text n_comments responses_has_comments ------------------------ id_responses id_comments comments ------------------------ id_comments text
the link table - reviews_has_comments (rhc) exists because comments can be added to several other types of content.
A cascading foreign key constraint on rhc removes rows when a comment or response has been deleted. This is working as expected. I have written a trigger to increment/decrement n_comments on responses when a change occurs in rhc.
The incrementing trigger is working fine, but the decrementing trigger is not. Here are the triggers.
CREATE TRIGGER `inc_n_comments_in_responses` AFTER INSERT ON `responses_has_comments` FOR EACH ROW BEGIN UPDATE responses SET n_comments = n_comments + 1 WHERE id_responses = NEW.id_responses; END; $$ CREATE TRIGGER `dec_n_comments_in_responses` AFTER DELETE ON `responses_has_comments` FOR EACH ROW BEGIN UPDATE responses SET n_comments = n_comments - 1 WHERE id_responses = OLD.id_responses; END; $$
Can anyone cast their eye over this and point me in the right direction? As I understand it, the BEFORE and AFTER commands happen before and after the CASCADE.
OK - this is a case of RTFM (although in this instance the manual sucks....)
MySQL documentation on this matter shows that Foreign Key constraints do NOT activate triggers.
So - let's look at the options:
1: I can increase the scope of the delete trigger thus:
CREATE TRIGGER `dec_n_comments` AFTER DELETE ON `comments` FOR EACH ROW BEGIN SELECT rshc.id_responses INTO @id_responses FROM responses_has_comments AS rshc WHERE id_comments = OLD.id_responses IF @id_responses IS NOT NULL THEN UPDATE responses SET n_comments = n_comments - 1 WHERE id_responses = @id_responses; END IF; -- rinse and repeat for all linked "commentable" table data... END; $$
My problem with this is that as the types of content to which comments can be added increases, so to will the size of this IF/ELSE (and it's bigger brother, the UPDATE trigger)
I could possibly remove the multiple IF/ELSE statements this points to if I write a PROCEDURE to return a placeholder table name and identifier to use in the subsequent UPDATE clause. This would require an additional field on the comments table in order to get the relevant table type - but I'm personally suspicious of referencing table meta data in this way...
I could possibly replace the resource/link/comment paradigm with a one to many relationship - suffixing each type of comment with it's parent resource: i.e response_comments
Finally - I could just sack this off and keep it in the application - which at the moment is really looking like the most likely option.