How create postgres trigger in rails?

Do you want to create a trigger in the Database so on insert the DB does the work?

http://www.postgresql.org/docs/9.1/static/sql-createtrigger.html

CREATE FUNCTION addRaitings() RETURNS TRIGGER AS '
   BEGIN
      INSERT INTO product_raitings(product_id,rate_type,rate) values(NEW.id,0,0);
      INSERT INTO product_raitings(product_id,rate_type,rate) values(NEW.id,1,0);
      INSERT INTO product_raitings(product_id,rate_type,rate) values(NEW.id,2,0);
      INSERT INTO product_raitings(product_id,rate_type,rate) values(NEW.id,3,0);
   END;
   ' LANGUAGE plpgsql;

CREATE TRIGGER createRaitingTrigger
        AFTER INSERT ON products
        FOR EACH ROW
        EXECUTE PROCEDURE addRaitings();

or do you want rails to do the work? then it would be an after_create callback http://api.rubyonrails.org/classes/ActiveRecord/Callbacks.html

so perhaps something like this.

class Product < ActiveRecord::Base
  has_many product_raitings

  after_create :build_default_raitings

 private
 def build_default_raitings
    (0..3).each { |x| self.product_raitings.create(:rate_type => x, :rate => 0) }
 end
end