How should I manage my many-to-many relationships?

Question

I have a database containing a couple tables: files and users. This relationship is many-to-many, so I also have a table called users_files_ref which holds foreign keys to both of the above tables.

Here's the schema of each table:

files -> file_id, file_name

users -> user_id, user_name

users_files_ref -> user_file_ref_id, user_id, file_id

I'm using Codeigniter to build a file host application, and I'm right in the middle of adding the functionality that enables users to upload files. This is where I'm running into my problem.

Once I add a file to the files table, I will need that new file's id to update the users_files_ref table. Right now I'm adding the record to the files table, and then I imagined I'd run a query to grab the last file added, so that I can get the ID, and then use that ID to insert the new users_files_ref record.

I know this will work on a small scale, but I imagine there is a better way of managing these records, especially in a heavy-traffic scenario.

I am new to relational database stuff but have been around PHP for a while, so please bear with me here :-)

I have primary and foreign keys set up correctly for the files, users, and users_files_ref tables, I'm just wondering how to manage the adding of file records for this scenario?

Thanks for any help provided, it's much appreciated.

-Wes

Answer 1

Use $this->db->insert_id() to get the id number of the row you just inserted. Further documentation here: http://codeigniter.com/user_guide/database/helpers.html

Answer 2

You're basically describing how it normally is done, with one important adjustment: how you retrieve the file_id of the file to be able to add it to users_files_ref.

Normally in a database environment you have many clients connecting at the same time, doing updates simultaneously. In such an environment you can't just get the file_id of the last file added - it might be someone elses file added in between your DB calls. You have to use functionality of the database to get the ID generated (e.g. SELECT @@IDENTITY on MSSQL) or generate the IDs in the application code somehow.

Answer 3

I think what you need is just this:

                   ----primary key-----
users_files_ref -> | user_id, file_id |

How you get the the file_id is dependent on the code you're implementing. Your reasoning is correct. You already have the user_id and just need to get the file_id. With these values you can add a new row to user_files_ref.

When I need to do this I usually have a stored procedure with the help of a sequence that inserts the file and returns the sequence NEXTVAL as the output. This might be a way of implementing such cenario.

This is the code for an Oracle based stored procedure:

CREATE OR REPLACE PROCEDURE SP_IMPORT_FILE(FILE    IN  FILE.FILE%TYPE,
                                           FILE_ID OUT NUMBER)

IS

BEGIN

  SELECT SEQ_FILE.NEXTVAL INTO FILE_ID from DUAL;

  INSERT INTO FILE (FILE_ID, FILE) VALUES (FILE_ID, FILE);

END SP_IMPORT_FILE;