Use NEW in FROM clause in Postgres trigger?
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21-01-2021 - |
Question
I am trying to write a Postgres trigger to unnest an array field before inserting or updating a new row. E.g.
SELECT
unnest(something)
FROM NEW
However, this seems to lead to an error:
relation 'new' does not exist
How can I use the NEW row inside of the trigger function, in such a way that allows me to unnest an array field for further processing?
CREATE TABLE statements
Below are examples of the table structures:
CREATE TABLE parent_table (
id uuid NOT NULL,
jsonb_array_field jsonb[] NOT NULL DEFAULT '{}'::jsonb[],
CONSTRAINT "parent_table_pkey" PRIMARY KEY (id),
);
CREATE TABLE many_to_one_table (
id serial primary key,
parent_table_id uuid references parent_table(id),
subfield_a text,
subfield_a text
);
TRIGGER function
Here is the essence of the TRIGGER function:
CREATE OR REPLACE FUNCTION unnest_and_normalize_json() RETURNS TRIGGER AS
$body$
begin
if NEW.jsonb_array_field is null then
raise exception 'jsonb_array_field is null';
else
insert into
many_to_one_table(subfield_a, subfield_b)
select
parent_table_id, -- this is to be the ForeignKey
json_data->>'subfieldA' as subfield_a,
json_data->>'subfieldB' as subfield_b
from (
select
id, -- need ID for ForeignKey relationship
unnest(jsonb_array_field) as json_data
from new
) as unnested_json;
end if;
RETURN new;
end;
$body$
LANGUAGE plpgsql;
TRIGGER statement
The trigger statement can be run either before or after INSERT and UPDATE, so long as the data are mirrored in the 'migration' table.
CREATE TRIGGER unnest_and_normalize_json_on_insert AFTER INSERT ON parent_table
FOR EACH ROW EXECUTE PROCEDURE unnest_and_normalize_json();
CREATE TRIGGER unnest_and_normalize_json_on_update AFTER UPDATE ON parent_table
FOR EACH ROW EXECUTE PROCEDURE unnest_and_normalize_json();
ForeignKey clarification
We are trying to transition this part of our data model to use a ForeignKey relationship rather than JSON field. The trigger is intended as a temporary step to ensure the normalized_table
has data going forward, while we backfill/migrate data from older records.
Solution
NEW
and OLD
in trigger functions are records - each only defined where applicable. (So no OLD
in an INSERT
trigger, for instance.) You can perform ROW
operations on them. Reference nested columns with dot notation (like table-qualified column names) like they were tables included in the FROM
list. NEW
and OLD
are visible almost everywhere in SQL DML statements in a trigger function. (Except dynamic SQL with EXECUTE
.)
To treat NEW
as actual table you'd have to transform it first - which is typically not necessary. Like: (SELECT NEW.*)
. For a meaningful use case see chapter 3. in this related answer:
Your basic trigger function burns down to:
CREATE OR REPLACE FUNCTION unnest_and_normalize_json()
RETURNS trigger AS
$func$
BEGIN
INSERT INTO many_to_one_table (parent_table_id, subfield_a, subfield_b)
SELECT NEW.id
, ja->>'subfieldA' AS subfield_a -- column alias only for documentation
, ja->>'subfieldB'
FROM unnest(NEW.jsonb_array_field) ja; -- produces a derived table
RETURN NEW; -- optional for AFTER trigger
END
$func$ LANGUAGE plpgsql;
This was just noise:
if NEW.jsonb_array_field is null then ...
The column jsonb_array_field
is defined as:
jsonb_array_field jsonb[] NOT NULL DEFAULT '{}'::jsonb[],
It's an AFTER
trigger, the NOT NULL
constraint would already have raised an exception for NULL
in jsonb_array_field
at this point.
You can merge your two triggers into one:
CREATE TRIGGER unnest_and_normalize_json_on_insert
AFTER INSERT OR UPDATE ON parent_table -- !
FOR EACH ROW EXECUTE PROCEDURE unnest_and_normalize_json();
That said, you'd have to do a lot more to cover UPDATE
and DELETE
properly. Similar to this one (chapter The dark side):