How to use PostgreSQL triggers? - ruby-on-rails

Question

I am trying to use PostgreSQL triggers in my rails app. So I tried using this migration where execution of triggers is supposedly easy:
-- class AddTriggersToProducts < ActiveRecord::Migration
def self.up
table :products
execute %q{
create trigger trig1 before insert on products for each row
begin
price = price + 5
end;
}
end
def self.down
execute 'DROP TRIGGER trig1'
end
end
But this didn't change anything. I don't know where to write the procedure or function if I am going to use one here ...

Answer 1

"Creating a trigger" consists of two steps in PostgreSQL:
1.) Create a trigger function - with special return value trigger:
CREATE FUNCTION trg_update_prod_price()
RETURNS trigger AS
$func$
BEGIN
NEW.price := NEW.price + 5;
RETURN NEW;
END
$func$ LANGUAGE plpgsql;
Multiple triggers can use the same trigger function.
2.) Create a trigger calling an existing trigger function:
CREATE TRIGGER update_prod_price
BEFORE INSERT ON products
FOR EACH ROW EXECUTE PROCEDURE trg_update_prod_price();
To "drop the trigger" (meaning the trigger function), you have to first drop all triggers referencing it and then drop the trigger function itself.
DROP TRIGGER update_prod_price ON products;
DROP FUNCTION trg_update_prod_price();
If you drop a table, all attached triggers are dropped with it. No need to drop those separately.

Answer 2

Does something like this work? Creating a function and then executing the function for the trigger:
def self.up
execute %q{
create or replace function update_price() returns trigger as $$
begin
NEW.price := NEW.price + 5;
return NEW;
end;
$$ language plpgsql }
execute %{ create trigger trig1 before insert on products for each row execute function update_price()}
end

Answer 3

The hair_trigger gem is a nice way to manage the creation of triggers.
Here is an example from hair_trigger's docs:
class AccountUser < ActiveRecord::Base
trigger.after(:insert) do
"UPDATE accounts SET user_count = user_count + 1 WHERE id = NEW.account_id;"
end
trigger.after(:update).of(:name) do
"INSERT INTO user_changes(id, name) VALUES(NEW.id, NEW.name);"
end
end