Equivalent plpgsql trigger in C

Hi.

I've a PostgreSQL 9.0 server and I'm using heritage on some tables, for this reason I have to simulate foreign keys through triggers like this:

CREATE OR REPLACE FUNCTION othertable_before_update_trigger()

RETURNS trigger AS
$BODY$
DECLARE
  sql   VARCHAR;
  rows  SMALLINT;
BEGIN

  IF (NEW.parenttable_id IS DISTINCT FROM OLD.parenttable_id) THEN

    sql  := 'SELECT id '
         || 'FROM parentTable '

         || 'WHERE id = ' || NEW.parenttable_id || ';';

     BEGIN
        EXECUTE sql;
        GET DIAGNOSTICS rows = ROW_COUNT;


     EXCEPTION
        WHEN OTHERS THEN
           RAISE EXCEPTION 'Error when I try find in parentTable the id %. SQL: %. ERROR: %',

                            NEW.parenttable_id,sql,SQLERRM;

     END;

     IF rows = 0 THEN
        RAISE EXCEPTION 'Not found a row in parentTable with id %. SQL: %.',NEW.parenttable_id,sql;

     END IF;
  END IF;

  RETURN NEW;
END;
$BODY$
LANGUAGE plpgsql;

But due to performance I try to create a equivalent trigger in C code:

#include "postgres.h"

#include "executor/spi.h"       /* this is what you need to work with SPI */

#include "commands/trigger.h"   /* ... and triggers */


#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif
extern Datum othertable_before_update_trigger(PG_FUNCTION_ARGS);


PG_FUNCTION_INFO_V1(othertable_before_update_trigger);

Datum

 othertable_before_update_trigger(PG_FUNCTION_ARGS) {

   TriggerData *trigdata = (TriggerData *) fcinfo->context;

   TupleDesc   tupdesc;
   HeapTuple   rettuple;

   bool        isnull;
   int         ret, i;


   /* make sure it's called as a trigger at all */
   if (!CALLED_AS_TRIGGER(fcinfo))

     elog(ERROR, "othertable_before_update_trigger: not called by trigger manager");


   /* tuple to return to executor */
   if (TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event))

     rettuple = trigdata->tg_newtuple;
   else

     rettuple = trigdata->tg_trigtuple;

   tupdesc = trigdata->tg_relation->rd_att;


   /* connect to SPI manager */
   if ((ret = SPI_connect()) < 0)

     elog(ERROR, "othertable_before_update_trigger (fired %s): SPI_connect returned %d", "before", ret);


   [A]

   [B]


   return PointerGetDatum(rettuple);
 }

I need fill the code in:

1. [A]: get the previous and new values for parenttable_id. With:

   int32 att = DatumGetInt32(heap_getattr(rettuple, 1, tupdesc, &isnull));

   or

   int32 att = DatumGetInt32(SPI_getbinval(rettuple, tupdesc, 1, &isnull));

I can get only the old value of parenttable_id but not the new value. Even if I try to use the column name instead of their number with:

GetAttributeByName (rettuple->t_data, "parenttable_id", &isnull);

Getting error: record type has not been registered

1. [B]: execute the query SELECT id FROM parentTable WHERE id = NEW.parenttable_id

I found the function SPI_execute_with_args, but I haven't found examples of this for my case.

Thanks in advance.