Connecting One table to Many tables - php

Question

I am building a commenting system where people can comment on uploaded files, messages and to-do items. What is the best way to connect the comment table table to the other various tables?
Possible Solutions
Solution one - use a two field foreign key.
CREATE TABLE `comments`(
id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
foreign_key INT NOT NULL,
table_name enum('files','messages','to-do'),
user_id INT NOT NULL,
comment TEXT NOT NULL);
Solution two - Each table would have a primary key unique to the database. So I would use php's uniqid($prefix) as the primary keys for each table.
CREATE TABLE `comments`(
id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
foreign_key char(23) NOT NULL,
table_name enum('files','messages','to-do'),
user_id INT NOT NULL,
comment TEXT NOT NULL);
Solution Three - Have multiple foreign keys in the comment table
CREATE TABLE `comments`(
id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
files_id INT NOT NULL,
messages_id INT NOT NULL,
to_do_id INT NOT NULL,
user_id INT NOT NULL,
comment TEXT NOT NULL);
What is the best solution? I appreciate your input and please let me know if I can clarify anything
EDIT removed table_name from solution three as it was a copy_paste error
As to Joe's Response
Assume:
1) all data is already escaped. Do we really need to see that?
2) $fileId = "146".
3) $userId = "432".
4) $comment = "Stackoverflow is so awesome!"
INSERT
$link = mysql_connect('localhost', 'mysql_user', 'mysql_password');
if (!$link) {
die('Could not connect: ' . mysql_error());
}
mysql_select_db('mydb');
mysql_query("INSERT INTO `comments` (user_id,comment) VALUES($userId,$comment)");
$commentId = mysql_insert_id();
mysql_query("INSERT INTO `comments_files_xref` (file_id,comment_id) VALUES($fileId,$commentId)");

Answer 1

Personally, I would normalize the design a bit more. Perhaps something like:

Answer 2

Multiple remarks :
You shouldn't call your foreign key foreign_key because a foreign key is a constraint, not a field in a way. it references a field in a table to an index on another, call it the same way you called the PK on the table you reference, or something recognizable.
Foreign keys constraints only works on innodb, if you use MyISAM forget about them and do a lot of checks with PHP.
read http://dev.mysql.com/doc/refman/5.0/en/innodb-foreign-key-constraints.html
You should make a sketch of what you want in your DB or use tools like mysql workbench (which is free) that helps seeing the schema better.
As I see your problem and if you want to use constraint here, I'll use solution one or another solution :
1-
CREATE TABLE `comments`(
id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, // Which is your comment index
idTable INT NOT NULL, // ID of the message
table_name enum('files','messages','to-do'), // which it comes from
user_id INT NOT NULL, // etc...
comment TEXT NOT NULL);
But there are conditions :
The PK of files, messages _to-do_ must have the same format (INT)
If you want to add a module (to files, messages _to-do_) it'll be difficult
2-
Create tables joining comments and other tables :
CREATE TABLE `comments`(
id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, // Which is your comment index
user_id INT NOT NULL, // etc...
comment TEXT NOT NULL);
CREATE TABLE `comments-files`(
id_comments INT NOT NULL PRIMARY KEY,
id_files INT NOT NULL PRIMARY KEY);
etc. Hope you see the point here. You add constraint thanks to http://dev.mysql.com/doc/refman/5.0/en/innodb-foreign-key-constraints.html if needed.

Answer 3

I just learn Ruby on Rails in my current company, and solution 1 is preferred because RoR's Active Record can handle it as polymorphic relation.
Back to the topic, that you are using PHP, I prefer either solution 1 or 3. Solution 1 is preferable if there are possibilities that the comment table will be used for other table in the future.
One note, in solution 3, I think the table_name column is not needed. You can determine for which table the comment is by fill either files_id, messages_id, or to_do_id with the id, then set 2 other foreign key with 0.

Answer 4

I would create a join table for each thing that can be commented on, so an files_comments table and a todo_comments table. But solution 1 would be an alternative. I would avoid solutions two or three... could get messy if things change in the future.