What exactly is the difference between a data mapper and a repository?

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Suppose your application manages Person objects, with each instance having name, age and jobTitle properties.

You would like to persist such objects, retrieve them from the persistence medium and maybe update (say, on their birthday, increment the age) or delete. These tasks are usually referred to as CRUD, from Create, Read, Update and Delete.

It is preferable to decouple your “business” logic from the logic that deals with the persistence of Person objects. This allows you to change the persistence logic (e.g. going from a DB to a distributed file system) without affecting your business logic.

You do this by encapsulating all persistence logic behind a Repository. A hypothetical PersonRepository (or Repository<Person>) would allow you to write code like this:

Person johnDoe = personRepository.get(p=> p.name == "John Doe");
johnDoe.jobTitle = "IT Specialist";
personRepository.update(johnDoe);

This is just business logic and doesn’t care about how and where the object is stored.

On the other side of the Repository, you use both a DataMapper and something that translates queries from the functional description (p=> p.name == "John Doe") to something that the persistence layer understands.

Your persistence layer can be a DB, in which case the DataMapper converts a Person object to and from a row in a PersonsTable. The query translator then converts the functional query into SELECT * FROM PersonsTable WHERE name == "John Doe".

Another persistence layer can be a file system, or another DB format that chooses to store Person objects in two tables, PersonAge and PersonJobTitle.

In the latter case, the DataMapper is tasked with converting the johnDoe object into 2 rows: one for the PersonAge table and one for the PersonJobTitle table. The query logic then needs to convert the functional query into a join on the two tables. Finally, the DataMapper needs to know how to construct a Person object from the query’s result.

In large, complex systems, you want to use small components that do small, clearly defined things, that can be developed and tested independently:

  • The business logic deals with a Repository when it wants to read or persist objects, and doesn’t care how that is implemented.
  • The Repository deals with a DataMapper when it wants to read/write an object in a particular persistence medium.
  • For querying, the Repository relies on a schema provided by the DataMapper (e.g. the jobTitle value is found in the JobTitle column in the PersonTable table) but not on any implementation of a mapper.
  • For DB persistence, the DataMapper relies on a DB layer, that shield it from the Oracle/Sybase/MSSQL/OtherProvider details.

The patterns don’t “differ”, they just expose different basic features.

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