Wait, what? PK is not needed for HABTM?

tomdonarski — Sun, 04 Jan 2026 08:16:28 +0000

While skimming over Rails’ docs I’ve found a sentence about HABTM (has_and_belongs_to_many) association that tickled my interest:

(…) For has_and_belongs_to_many associations, you'll need to create a join table without a primary key.

It made my mind go blank for a moment, and I started to wonder "Hey, can a table do not have a primary key?"

👉 Yes, it's possible. Primary key is just a constraint (source).

Then I went on to inspect an immediate project to see if there’s any join table for HABTM association, and saw this:

CREATE TABLE public.organizations_vendors (
    id integer NOT NULL,
    organization_id integer,
    vendor_id integer
);

I started digging, and found that the HABTM join table is an implementation detail, not a real model. Which is also hinted by Rails’ docs:

You'd use has_and_belongs_to_many when:

The association is simple and does not require additional attributes or behaviors on the join table.

You do not need validations, callbacks, or extra methods on the join table.

It turns out PK on a HABTM join table is not required, as functionally it’s irrelevant.

If it is (application-wise) - then most likely has_many :through would be a better choice (source).

You'd use has_many :through when:

You need to add extra attributes or methods to the join table.

You require validations or callbacks on the join model.

The join table should be treated as an independent entity with its own behavior.

Forem: tomdonarski

Wait, what? PK is not needed for HABTM?