Re: Multi-table insert using RULE - how to handle id? - Mailing list pgsql-sql

Aaron Bono wrote:

> On 7/19/06, *Collin Peters* <cadiolis@gmail.com 
> <mailto:cadiolis@gmail.com>> wrote:
>
>     I am learning about how to use rules to handle a multi-table insert.
>     Right now I have a user_activity table which tracks history and a
>     user_activity_users table which tracks what users are associated with
>     a row in user_activity (one to many relationship).
>
>     I created a rule (and a view called user_activity_single) which is to
>     simplify the case of inserting a row in user_activity in which there
>     is only one user in user_activity_users.
>
>     CREATE OR REPLACE RULE user_activity_single_insert AS
>     ON INSERT TO user_activity_single
>     DO INSTEAD (
>             INSERT INTO user_activity(
>                     user_activity_id,
>                     description,
>                     ...
>             )
>             VALUES (
>                     NEW.user_activity_id,
>                     NEW.description,
>                     ...
>             );
>             INSERT INTO user_activity_users (
>                     user_activity_id,
>                     user_id
>             )
>             VALUES (
>                     NEW.user_activity_id,
>                     NEW.user_id
>             );
>     );
>
>     This works well by itself, but the problem is that I have to manually
>     pass in the user_activity_id which is the primary key.  I do this by
>     calling nextval to get the next ID in the sequence.
>
>     Is there any way to have the rule handle the primary key so I don't
>     have to pass it in?  It seems you can't use pgsql inside the rule at
>     all.  What I'm looking for is something like:
>
>     CREATE OR REPLACE RULE user_activity_single_insert AS
>     ON INSERT TO user_activity_single
>     DO INSTEAD (
>             SELECT nextval('user_activity_user_activity_id_seq') INTO
>     next_id;
>
>             INSERT INTO user_activity(
>                     user_activity_id,
>                     description,
>                     ...
>             )
>             VALUES (
>                     next_id,
>                     NEW.description,
>                     ...
>             );
>             INSERT INTO user_activity_users (
>                     user_activity_id,
>                     user_id
>             )
>             VALUES (
>                     next_id,
>                     NEW.user_id
>             );
>     );
>     Note the sequence stored in next_id.  This doesn't work as it
>     complains about next_id in the INSERT statements.  Any way to do
>     something like this?  I suppose I could create a function and then
>     have the rule call the function but this seems like overkill. 
>
>
>  
> Since I have not tried something like this before, I may be off base 
> but have you tried:
>
> CREATE OR REPLACE RULE user_activity_single_insert AS
>  ON INSERT TO user_activity_single
> DO INSTEAD (
>        INSERT INTO user_activity(
>                description,
>                ...
>        )
>        VALUES (
>                NEW.description,
>                ...
>        );
>        INSERT INTO user_activity_users (
>                user_activity_id,
>                user_id
>        )
>        VALUES (
>                SELECT currval('user_activity_user_activity_id_seq'),
>                NEW.user_id
>        );
> );
>
> I am assuming user_activity.user_activity_id is a BIGSERIAL or SERIAL.

By coincidence, I had to work this out just the other day for a project 
that I'm working on. Aaron's solution looks correct, although you can 
just call currval() without the SELECT in this context.

What Collin may not be aware of is that currval(seqX) returns the last 
value generated by nextval(seqX) in the same session, so there is no 
race between your session and other sessions that may be using the same 
sequence. So it's safe to let the insert trigger the next value 
automatically, and then call currval(). In contrast, don't use lastval() 
for this.

Ross Johnson