More often than not, you aren't porting. You are making the system work with more than one kind of database. When my first company was acquired and was built on PostgreSQL (which was designed to be hosted) the acquirer wanted an on-premise version that also supported Oracle. Using an ORM made that a relatively simple process with only a few annoying differences between the two databases. Specifically, treatment of strings (oracle empty string is a null string), booleans (no boolean type in oracle) and DDL (oracle doesn't have transactional DDL).