when to set a table without primary key on a join table

Hey I am using the following tables. My table course_enrollments references the user_id and the course_id. Both columns combined are set unique.

users

course_enrollments

courses

Rails sets primary keys by default. I have read some articles of people who used these kind of join tables without any pk. e.g. by defining :id => false

Is it appropriate for this kind of join table? what about people enrolling or leaving courses or the search for enrollments. Thanks for your time!

You generally see :id => false on the migration for your join table when you want to use has_and_belongs_to_many, since Rails really doesn't like finding an :id primary key on these.

From the has_and_belongs_to_many docs

The join table should not have a primary key or a model associated with it.

Both columns combined are set unique

Therefore your table already has a key. Unless you require other attributes there is no obvious need to make another key for it.