Mysql – Derived value not calculating as expected using combination of FK and Triggers (MySQL)

foreign keyMySQLtrigger

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.

Best Answer

reviews_has_comments (rhc) exists because comments can be added to several other types of content.

If I understand correctly, that is not a justification for responses_has_comments table - you can have several tables with 1-to-many relationships with the same comments table. The only reason for introducing responses_has_comments would be if the same comment can be attached to more than one response, as well as more than one comment being attached to each response, ie it is a many-to-many relationship - I don't think that is what you are trying to achieve.

This does not answer your main question but may sidestep the problem in your updated trigger. Your comments trigger then becomes:

CREATE TRIGGER `dec_n_comments`
    AFTER DELETE ON `comments` FOR EACH ROW
    BEGIN
        UPDATE responses
        SET n_comments = n_comments - 1
        WHERE id_responses = @id_responses;
        -- rinse and repeat for all linked "commentable" table data...        
    END;
$$
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