I know this is an old thread, but as it seems to have the best advice
I've seen on this subject, I'm going to throw my few cents in here.
I'm strongly leaning in the direction of a shared primary/foreign key
for situations where the parent record is a prerequisite for the child
record. Effectively, the child record is an aspect of the parent
record. It could be modeled using a single parent table that is really
wide (i.e. that contains columns for all of the child records), but
there may be good reasons not to do that.
The approach I am taking is as follows:
Use the normal Rails :id column for both tables.
It appears in testing against Postgresql that one can have an
auto-increment primary key and still specify the primary key value when
creating a new record. In order to enforce this, I specify `validates
:id, presence: true, uniqueness: true` in the model for the child table.
If I run into a database that won't let me specify the value for an
auto-increment field when creating a new record I'll switch to using
`id: false` in the migration and manually create the `id` field as an
integer and then specify a unique index on it.
I specify belongs_to on the child record and pass `foreign_key: :id`. I
do the same for specifying has_one on the parent record.
There is no need to specify self.primary_key (the replacement for
set_primary_key) on the child model since the primary key is still :id.
I'm using a nested singular resource for the child record when routing
like so:
resources :user do
resource :instructor
end
That sets up routes like so:
/users/:id
/users/:user_id/instructor
Because I used :id throughout, when using the route helpers like
user_instructor_path, I can pass either a User or an Instructor object
and I will still get the correct route.
I'm still experimenting with this approach, but so far I'm reasonably
happy with it.
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