How to use explode mysql function in where condition with cakephp
by Unique constraint on your payment_method field, you can make sure that payment_method is unique. If the peyment_method is already exists, it will throw a SQL Error.


  Duplicate entry 'x' for key 'peyment_method'


The SQL for that would be:

ALTER TABLE YourTableName ADD UNIQUE (payment_method)


But if you mean you need to make sure this payment_method exists and the payment_method_id is in another table, you need foreign key.

ALTER TABLE YourTable ADD FOREIGN KEY `methodidCheck`(payment_method)
REFERENCES payment_method_table(payment_method_id);


Hope I got your question right