MySQL foreign key problem rails - mysql

Question

I'm trying to add foreign keys to my table using the following code:
constraint_name = "fk_#{from_table}_#{to_table}"
execute %{
CREATE TRIGGER #{constraint_name}_insert
BEFORE INSERT ON #{from_table}
FOR EACH ROW BEGIN
SELECT
RAISE(ABORT, "constraint violation: #{constraint_name}")
WHERE
(SELECT id FROM #{to_table} WHERE
id = NEW.#{from_column}) IS NULL
END
}
I'm getting the following error:
Mysql2::Error: Not allowed to return a result set from a trigger:
CREATE TRIGGER fk_brands_products_insert
BEFORE INSERT ON brands
FOR EACH ROW BEGIN
SELECT
RAISE(ABORT, "constraint violation: fk_brands_products")
FROM brands
WHERE
(SELECT id FROM products WHERE id = NEW.brand_id) IS NULL;
END;
What is wrong with my SQL script?

Answer 1

You can add a MySQL Foreign Key constraint like this:
ALTER TABLE #{from_table}
ADD CONSTRAINT #{constraint_name} FOREIGN KEY
(#{from_column}) REFERENCES #{to_table}(id)
This will constrain from_column on from_table to values in id in #{to_table}.
P.S: If you don't want to name the CONSTRAINT you can do this:
ALTER TABLE #{from_table}
ADD FOREIGN KEY
(#{from_column}) REFERENCES #{to_table}(id)

Answer 2

As the error states, a trigger is not allowed to return a result set. Even though you are raising a error, MySql doesn't like the SELECT statement. Try using a IF:
IF EXISTS(SELECT id FROM #{to_table} WHERE
id = NEW.#{from_column}) = 0 THEN
RAISE(ABORT, "constraint violation: #{constraint_name}");
END IF;