Question

What is the correct way to design this database?

Here is how I have my tables set up:

I have a many to many relationship between a table called teachers and a table called instruments. I then have a bridge table connecting the two. I would like to associate another table with the BRIDGE table. Meaning the instrument/teacher combination. That table would have 3 rows specifying what level of playing a teacher can teach (ie. Beginner, intermediate, advanced). Seems like I should set up the bridge table to have teacher_id, instrument_id AND level_id, but I don’t know if this is the conventional way to do it.

I am using mysql and cakephp, and I haven’t found anything in the documentation about the HABTM associations about having an extra field in the bridge table. Just want to make sure I’m doing it correctly.

Answer 1

If that’s what your data calls for, and it sounds like it does, then go for it.

Basically, as you noted, you’d have a three-part key:

• teacher_id
• instrument_id
• level_id

Which means that a teacher could teach guitars at beginner and advanced, piano at intermediate and beginner, and another teacher could teach piano at all three levels and oboe at beginner … sounds good.

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Incidentally, the bridge table is also known as an intersect or intersection table.