Rails Postgres Insert Duplicate Row Check Issues - ruby-on-rails

Question

I'm trying to detect and silently discard any duplicate INSERTs into a table, and if a dupe detected, return the ID (Primary Key) of the existing record. Otherwise, insert the record and return the new ID.
I can do this with either a RULE or a TRIGGER, however both have drawbacks. Here's an example of my RULE:
CREATE OR REPLACE RULE territory_products_ignore_duplicate_inserts AS
ON INSERT TO territory_products
WHERE (EXISTS ( SELECT 1
FROM territory_products tp
WHERE tp.territory_id = NEW.territory_id AND tp.product_id = NEW.product_id)) DO INSTEAD SELECT tp.id
FROM territory_products tp WHERE tp.territory_id = NEW.territory_id AND tp.product_id = NEW.product_id LIMIT 1;
Testing this in a SQL console or psql with an INSERT works fine. If there's a dupe, it'll return the first existing record's id and not do the INSERT. Otherwise, it will go ahead with the INSERT. However, in Rails, it fails and returns this error:
ERROR: cannot perform INSERT RETURNING on relation "territory_products"
HINT: You need an unconditional ON INSERT DO INSTEAD rule with a RETURNING clause.
Moving to a TRIGGER, I try this:
CREATE OR REPLACE Function territory_products_ignore_dups() Returns Trigger
As $$
Begin
If Exists (
Select id From territory_products tp
Where tp.territory_id = NEW.territory_id And tp.product_id = NEW.product_id
) Then
Return NULL;
End If;
Return NEW;
End;
$$ Language plpgsql;
Create Trigger territory_products_ignore_dups
Before Insert On territory_products
For Each Row
Execute Procedure territory_products_ignore_dups();
This also works fine, except that I can't get it to return the existing ID, because of the Return NULL (which is required to disallow the INSERT).
Can anyone resolve either of these issues, so I get the result I'm looking for? (e.g. silently discard the INSERT in case of a dupe and return the ID of the existing record. Or if the INSERT is successful, return the ID of the new record).

Answer 1

Add a unique index to your database:
# migration file
add_index : territory_products, [:territory_id, :product_id], unique: true
In your logic look for the existing record or create if doesn't exist. It will raise an exception in case of a race condition. Just search again for the record.
#logic when finding/creating record
begin
TerritoryProduct.where(territory_id: params[:territory_id], product_id: params[:product_id]).first_or_create!
rescue ActiveRecord::RecordNotUnique
# In case it already exists
TerritoryProduct.where(territory_id: params[:territory_id], product_id: params[:product_id]).first
end