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. 

**UPDATE**

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.

http://dev.mysql.com/doc/refman/5.0/en/innodb-foreign-key-constraints.html

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.