Question

There are two tables – posts and comments:

create table posts
(
    id integer not null primary key auto_increment,
    body text not null
);

create table comments
(
    id integer not null primary key auto_increment,
    body text not null,
    post_id integer not null references posts(id)
);

Now I want to create one more table – reports(“bad post” flags) and I want it to store reports for both posts and comments.

create table reports
(
    id integer not null primary key auto_increment,
    obj_type tinyint not null, /* '1' for posts and '2' for comments */
    obj_id integer not null,
    body text not null
);

alter table reports add foreign key(obj_id) references posts(id) on delete cascade;
alter table reports add foreign key(obj_id) references comments(id) on delete cascade;

As you see there are two references in a single field (I differentiate them by obj_id), and the question is – Is it all right to do like this ?

If not what would be better solution?

Thanks in advance.

Answer 1

Intuitively that feels not the right way to do it. I think MySQL would be confused too; how would it validate that a constraint is being met; would it try posts first, or comments first … maybe both?

Personally I would choose to create two link tables:

• comments <-> reports
• posts <-> reports

That way you disambiguate the obj_id properly.