Reporting errors when a rule fails.

I have posted this question in the past and did not receive a response.

> Reporting errors when a rule fails.
>
> I am currently design a database to be implemented using PostGreSQL 6.5.1.
> I have two tables with the following schema:
>
> create table1 ( myId : serial, status int4 );
>
> create table2 ( myId : serial, parentId : int4, status );
>
parentId is a foreign key into table1.

> I created a function that checks to make sure that a parent exists in
> table1. Not sure if this is the best way, suggestions welcome.
>
> create function parent_exists(int4) returns boolean as 'select
> int4eq(count(*), 1) from table1 where table1.myId = $1' language 'sql'
>
> I then created a rule to reject insertions into table2 if a parent does
> not exists in table1.
>
> create rule validate_parent_exists as on insert to table2 where
> parent_exists(new.parentId) = false do instead nothing
>
> Is there a way to return an error to the caller, who attempted the insert
> into table2 where the parent does not exists in table1?
>
> As mentioned, if there is a better way to implement this functionality,
> suggestions are welcome.
>
> Thanks,
> Scott Saltsgaver
> Applied Innovation Inc.
> scottsa(at)aiinet(dot)com