Do I need a join table for a has_many :through association?

Question

I've only created a has_and_belongs_to_many association before and it is different from has_many :through. For a has_many :through association, do I need a join table? How does the actual association work? Do I need an index? I can't find a great tutorial on this, any suggestions?

Answer 1

Yes you need the join table. This shows how: http://railscasts.com/episodes/47-two-many-to-many

This may also be helpful; has_many :through questions

By the way if you need to search with condition this will help: Has many through associations with conditions

Also a great example with code of editing the nested attributes of the join table at Rails nested form with has_many :through, how to edit attributes of join model? .

All these are sorts of things you might find yourself wanting to do :)

Index are optional and also vary by db. mySQL used to only support 1 at a time. Not sure if that has changed.

Answer 2

It depends on how you want to use the has_many :through relation. There are two different cases (named in the guide to relations

1. for an n:m relation: the example in the guide is physicians have appointments with patients.
2. for an 1:n and an additional 1:n relation: the example in the guide is 1 document has many sections and 1 section has many paragraphs.

For the first one, you need the join table, you don't need the index. For the second one, you need none of them.