Updatable view and default sequence values

[Kouber Saparev <postgres@xxxxxxxxxxx>]

Hi All,

I am currently using PostgreSQL 8.1.3 and am trying to create an 
updatable view with two (or more) joined tables and I also would like to 
have the ability to indicate implicitly the value of the serial primary 
key fields.

I have the following two tables:

CREATE TABLE purchase (
  purchase_sid SERIAL PRIMARY KEY,
  data TEXT
);

CREATE TABLE subscription_purchase (
  subscription_purchase_sid SERIAL PRIMARY KEY,
  purchase_sid INT NOT NULL UNIQUE REFERENCES purchase ON UPDATE 
CASCADE ON DELETE CASCADE,
  data TEXT
);


I have also created the following view:

CREATE VIEW
  s_purchase AS
SELECT
  p.purchase_sid,
  p.data AS pdata,
  sp.subscription_purchase_sid,
  sp.data AS sdata
FROM
  purchase p INNER JOIN subscription_purchase sp
ON
  sp.purchase_sid = p.purchase_sid;


Now, in order to make the view updatable I added this rule:

CREATE RULE s_purchase_update AS
  ON UPDATE TO s_purchase DO INSTEAD (
    UPDATE
      purchase
    SET
      purchase_sid = NEW.purchase_sid,
      data = NEW.pdata
    WHERE
      purchase_sid = OLD.purchase_sid;

    UPDATE
      subscription_purchase
    SET
      subscription_purchase_sid = NEW.subscription_purchase_sid,
      purchase_sid = NEW.purchase_sid,
      data = NEW.sdata
    WHERE
      subscription_purchase_sid = OLD.subscription_purchase_sid;
);


The tricky part comes when I try to make my view insertable. Normally 
I'd insert without specifying the sequence values, but in some cases I'd 
want also to have full control of what's going into the underlying 
tables. The thing is that when I try to do it the simple way by 
specifying default values in the view itself:

ALTER TABLE s_purchase ALTER COLUMN purchase_sid SET DEFAULT 
NEXTVAL('purchase_purchase_sid_seq');
ALTER TABLE s_purchase ALTER COLUMN subscription_purchase_sid SET 
DEFAULT NEXTVAL('subscription_purchase_subscription_purchase_sid_seq');

CREATE RULE s_purchase_insert AS
  ON INSERT TO s_purchase DO INSTEAD (
    INSERT INTO purchase
      (purchase_sid, data)
    VALUES
      (NEW.purchase_sid, NEW.pdata);

    INSERT INTO subscription_purchase
      (subscription_purchase_sid, purchase_sid, data)
    VALUES
      (NEW.subscription_purchase_sid, NEW.purchase_sid, NEW.sdata);
);

I get foreign key constraint violation. That's happening because default 
values are executed *before* the rule, so NEXTVAL for the sequence 
'purchase_purchase_sid_seq' is executed twice - once for each table.

The work around is to remove the default value for this sequence and to 
call it in the rule itself with coalesce:

ALTER TABLE s_purchase ALTER COLUMN purchase_sid DROP DEFAULT;

CREATE RULE s_purchase_insert AS
  ON INSERT TO s_purchase DO INSTEAD (
    INSERT INTO purchase
      (purchase_sid, data)
    VALUES
      (COALESCE(NEW.purchase_sid, 
NEXTVAL('purchase_purchase_sid_seq')), NEW.pdata);

    INSERT INTO subscription_purchase
      (subscription_purchase_sid, purchase_sid, data)
    VALUES
      (NEW.subscription_purchase_sid, COALESCE(NEW.purchase_sid, 
CURRVAL('purchase_purchase_sid_seq')), NEW.sdata);
);

The thing is that in the real case I have multiple tables that have to 
be joined so I really want to get rid of all this COALESCE stuff and to 
put everything in the view definition.

Any ideas how to suppress multiple invocations of nextval() or how to do 
it anyway? :)

--
Kouber Saparev
http://kouber.saparev.com