What is Data Mapping?

Object Relational Mapping (ORM for short) is to automatically persist objects in an object-oriented language program to a relational database by using metadata describing the mapping between the object and the database. Essentially, transforming data from one form to another. This also implies additional execution overhead; however, if ORM is implemented as a middleware, there will be many opportunities for optimization, which are not present in the handwritten persistence layer. More importantly, the metadata used to control the transformation needs to be provided and managed; but again, these costs are less than maintaining a handwritten solution; and even object databases that adhere to the ODMG specification still require class-level metadata.