How to delimit PostgreSQL trigger procedure name? - postgresql

Question

For example, in Perl you delimit a variable like so:
${foo}_bar
I have a trigger in PostgreSQL borrowed from here that I am trying to make generic to work with multiple tables. Here's my code:
CREATE OR REPLACE FUNCTION update_parent_path() RETURNS TRIGGER AS $$
DECLARE
PATH ltree;
BEGIN
IF NEW.parent_id IS NULL THEN
NEW.parent_path = 'root'::ltree;
ELSEIF TG_OP = 'INSERT' OR OLD.parent_id IS NULL OR OLD.parent_id != NEW.parent_id THEN
SELECT parent_path || TG_TABLE_NAME_id::text FROM TG_TABLE_NAME WHERE TG_TABLE_NAME_id = NEW.parent_id INTO PATH;
IF PATH IS NULL THEN
RAISE EXCEPTION 'Invalid parent_id %', NEW.parent_id;
END IF;
NEW.parent_path = PATH;
END IF;
RETURN NEW;
END;
$$ LANGUAGE plpgsql;
Each table that I am using this trigger against has a primary key like table_id (e.g., skill_id, level_id, etc). What I am trying to do is say WHERE skill_id = NEW.parent_id (for whatever table it's called against), thus the reason I am saying WHERE TG_TABLE_NAME_id = NEW.parent_id. What I'm wondering is how do I delimit TG_TABLE_NAME (trigger procedure) from _id?
Or, is there a better way to do this? Maybe I'm just going about this all wrong.

Answer 1

PLpgSQL has one fundamental rule - plpgsql variable cannot be used as table name or column name in embedded SQL. But there are a dynamic SQL - next way, how to execute SQL query. Dynamic SQL is a query generated in runtime from string (or string expression). There a PLpgSQL variable can be used everywhere. So your query fragment:
SELECT TG_TABLE_NAME_id::text FROM TG_TABLE_NAME ...
is wrong in more points, and should not work ever. But the dynamic query (PLpgSQL statement EXECUTE) should to work
EXECUTE format('SELECT %I FROM %I ...',
TG_TABLE_NAME || '_id', TG_TABLE_NAME) INTO path;
Related documentation: http://www.postgresql.org/docs/9.4/static/plpgsql-statements.html#PLPGSQL-STATEMENTS-EXECUTING-DYN