Postgresql – foreign key violation within trigger on insert of ‘new’ id in other table as foreign key

postgresqltrigger

Within a trigger I am inserting the id of the table the trigger is assigned to as a foreign key in another table, but the 'new' id doesn't exist yet, so I get a foreign key constraint violation.

Here is a simplified version:

  CREATE or replace FUNCTION trg_func()
            RETURNS trigger
            LANGUAGE 'plpgsql'
        AS $BODY$
        DECLARE

        BEGIN

        INSERT INTO other_tbl (
        my_tbl_fk
        ,my_calc
        )
    SELECT new.id -- <- this one is the issue. Is fk in other_tbl & pk in my_tbl
        ,a.some_val / b.some_val calc
    FROM (...subquery here...) a
        ,(...subquery here...) b
    WHERE a.some_id = b.some_id;

        RETURN new ;
        END ;
        $BODY$ ;

        CREATE TRIGGER my_tbl_ins_upd_trg
            BEFORE INSERT OR UPDATE 
            ON my_tbl
        FOR EACH ROW
        EXECUTE PROCEDURE trg_func();

I tried temporarily removing the fk before the insert then adding it back, but it doesn't seem to help.

I can remove the fk, trigger the trigger, then add the fk back again, so I know my insert values don't violate the constraint if applied generally.

I am guessing there is a method or strategy for doing this because it seems like something that would be done a lot for calculated fields via trigger.

I am trying to do the 'Discarding 3NF With Triggers' section of this link:
http://database-programmer.blogspot.com/2008/01/database-skills-third-normal-form-and.html

PostgreSQL 9.3

Best Answer

That should be simply achieved by setting the foreign key constraints to be checked at the end of the transaction. This is achieved by means of a SET CONSTRAINTS ALL DEFERRED (or, at least, SET CONSTRAINTS name_of_fk_constraint DEFERRED.

As per PostgreSQL (9.3) documentation about SET CONSTRAINTS:

SET CONSTRAINTS sets the behavior of constraint checking within the current transaction. IMMEDIATE constraints are checked at the end of each statement. DEFERRED constraints are not checked until transaction commit. Each constraint has its own IMMEDIATE or DEFERRED mode.

If you want to use this feature, you need to create your constraints with the DEFERRABLE option. If you intend to use them mostly in deferred mode, it is a good idea to make them INTIALLY DEFERRED. By default, constraints are NOT DEFERRABLE.

It is advisable to make constraint names unique (PostgreSQL doesn't force you to; they only need to be unique per table), and to create the names explictily. This way, you can control its deferrability with guarantee that you always specify the right one.

If you want a specific FK constraint to behave already as DEFERRED, you can declare it with:

ALTER TABLE table_a
    ADD CONSTRAINT table_a_col_name_table_b_fk FOREIGN KEY (col_name)
    REFERENCES table_b (col_name)
    ON UPDATE NO ACTION   /* or whichever action is deemed appropriate */
    ON DELETE NO ACTION   /* or whichever action is deemed appropriate */
    DEFERRABLE INITIALLY DEFERRED;

This way, the FK constraint is checked, by default, at the end of the transaction, and the SET CONSTRAINT isn't needed.

Related Solutions

Postgresql – check key if exists in other table without fk constraint

I do agree with Aaron, that not creating a PK (or unique constraint) is a very strange requirement

But you could do something like this:

insert into b (id_b, id_a)
select 1, 2 
where exists (select 1 from a where id_a = 2);

or alternatively:

insert into b (id_b, id_a)
select 1, id_a 
from a where id_a = 2

If you want to insert multiple rows, you can use a values clause:

insert into b (id_b, id_a)
select * 
from (
   values 
      (1,2), (2,3), (3,4)
) as t(id_b, id_a) 
where exists (select 1  
              from a 
              where a.id_a = t.id_a);

This will only make sure that this specific insert will insert correct data. It will not prevent others that don't follow this pattern to insert invalid data. And it is not safe in a multi-user environment where different transactions run the inserts concurrently.

PostgreSQL: trigger to autopopulate other fields with sub-strings of other field inserted

The required trigger is quite fancy. This is the trigger function:

CREATE FUNCTION wh_calc_ins_upd_trg_func()
    RETURNS trigger
    LANGUAGE 'plpgsql'
AS $BODY$
DECLARE
    t text[] ;
BEGIN    
    SELECT array_agg(x[1]) 
    FROM regexp_matches(new.calc_form, '\[bp(\d+)\]', 'gi') AS x 
    INTO t ;

    new.bbg_pulls_1_id := t[1]::integer ;
    new.bbg_pulls_2_id := t[2]::integer ;
    new.bbg_pulls_3_id := t[3]::integer ;
    new.bbg_pulls_4_id := t[4]::integer ;
    new.bbg_pulls_5_id := t[5]::integer ;
    new.bbg_pulls_6_id := t[6]::integer ;
    new.bbg_pulls_7_id := t[7]::integer ;
    new.bbg_pulls_8_id := t[8]::integer ;

    SELECT array_agg(x[1]) 
    FROM regexp_matches(new.calc_form, '\[wc(\d+)\]', 'gi') AS x 
    INTO t ;

    new.wh_calc_1_id := t[1]::integer ;
    new.wh_calc_2_id := t[2]::integer ;
    new.wh_calc_3_id := t[3]::integer ;
    new.wh_calc_4_id := t[4]::integer ;
    new.wh_calc_5_id := t[5]::integer ;
    new.wh_calc_6_id := t[6]::integer ;
    new.wh_calc_7_id := t[7]::integer ;
    new.wh_calc_8_id := t[8]::integer ;

    RETURN new ;
END ;
$BODY$ ;

And this is the association of this function to a trigger that should be fired before insert or update of column 'calc_form'. [or...]

CREATE TRIGGER wh_calc_ins_upd_trg
    BEFORE INSERT OR UPDATE OF calc_form
    ON wh_calc
    FOR EACH ROW
    EXECUTE PROCEDURE wh_calc_ins_upd_trg_func();

This trigger plays with a few concepts:

Regular Expressions to capture the integers in [bp___] and [wc___]. The regexp_matches function returns a SET of text[].
The SET of text[] is converted into an array of arrays by means of array_agg.
We convert from text to integer, by means of tx::integer.

You can try then:

INSERT INTO 
    wh_calc
    (calc_form)
VALUES
    ('[bp20]/[bp3]') 
RETURNING 
    * ;

And see that you get what you expect:

| id |    calc_form | bbg_pulls_1_id | bbg_pulls_2_id | bbg_pulls_3_id | bbg_pulls_4_id | bbg_pulls_5_id | bbg_pulls_6_id | bbg_pulls_7_id | bbg_pulls_8_id | wh_calc_1_id | wh_calc_2_id | wh_calc_3_id | wh_calc_4_id | wh_calc_5_id | wh_calc_6_id | wh_calc_7_id | wh_calc_8_id |
|----|--------------|----------------|----------------|----------------|----------------|----------------|----------------|----------------|----------------|--------------|--------------|--------------|--------------|--------------|--------------|--------------|--------------|
|  4 | [bp20]/[bp3] |             20 |              3 |         (null) |         (null) |         (null) |         (null) |         (null) |         (null) |       (null) |       (null) |       (null) |       (null) |       (null) |       (null) |       (null) |       (null) |

You can check it a SQLFiddle

Best Answer

Related Solutions

Postgresql – check key if exists in other table without fk constraint

PostgreSQL: trigger to autopopulate other fields with sub-strings of other field inserted

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