PostgreSQL use NEW in query for INSTEAD OF trigger

I'm having trouble getting an INSTEAD OF trigger to work correctly, and I think I've misunderstood how to use NEW. Consider the following simplified scenario:

CREATE TABLE Product (
  product_id SERIAL PRIMARY KEY,
  product_name VARCHAR
);
CREATE TABLE Purchase (
  purchase_id SERIAL PRIMARY KEY,
  product_id INT REFERENCES Product,
  when_bought DATE
);

CREATE VIEW PurchaseView AS
SELECT purchase_id, product_name, when_bought
FROM Purchase LEFT JOIN Product USING (product_id);

I'd like to be able to create INSTEAD OF triggers to allow me to insert directly into PurchaseView, e.g.:

INSERT INTO Product(product_name) VALUES ('foo');
INSERT INTO PurchaseView(product_name, when_bought) VALUES ('foo', NOW());

What I had in mind was something along the lines of:

CREATE OR REPLACE FUNCTION insert_purchaseview_func()
  RETURNS trigger AS
$BODY$
BEGIN
  INSERT INTO Purchase(product_id, when_bought)
  SELECT product_id, when_bought
  FROM NEW 
  LEFT JOIN Product USING (product_name)
  RETURNING * INTO NEW;
END;
$BODY$
LANGUAGE plpgsql;

CREATE TRIGGER insert_productview_trig
  INSTEAD OF INSERT
  ON PurchaseView
  FOR EACH ROW
  EXECUTE PROCEDURE insert_purchaseview_func();

However the above trigger function gives errors (relation "new" does not exist) when executed. I know I can write queries that explicitly use attributes from NEW in the WHERE and SELECT clauses, but sometimes it would be convenient to be able to include NEW in a join. Is there a way to do this?

Current (unsatisfactory) solution

The closest I can get to what I want is

CREATE OR REPLACE FUNCTION insert_purchaseview_func()
  RETURNS trigger AS
$BODY$
DECLARE
tmp RECORD;
BEGIN
    WITH input (product_name, when_bought) as (
       values (NEW.product_name, NEW.when_bought)
    ) 
    INSERT INTO Purchase(product_id, when_bought)
    SELECT product_id, when_bought
    FROM input
    LEFT JOIN Product USING (product_name)
    RETURNING * INTO tmp;
    RETURN NEW;
END;
$BODY$
LANGUAGE plpgsql;

This is a little unsatisfactory for several reasons:

1. I need to explicitly write all the attributes of NEW in CTE WITH query, which for large views (especially those whose attributes are automatically determined with SELECT *) gets unwieldy;

2. The returned result doesn't have the SERIAL type product_id updated, so you don't get the expected result for

   INSERT INTO PurchaseView(product_name, when_bought) VALUES ('foo', NOW()) RETURNING *;