Data Mapper, Repository and Unit of Work patterns

Question

I'm learning about the following patterns

Data Mapper
Repository
Unit of Work

I think I understand each of them but I can't answer these questions

are they always used to together?
which pattern uses the others?
which pattern is known by the Domain Model?
what CRUD operations are handled by each of them?
who owns the database code (i.e. SQL)?

Thanks.

garethb garethb · Accepted Answer · 2016-11-28T05:07:47

Data Mapper will map your database objects to real world objects used in your application and back the other way (from real work objects to database objects). You would use this as you may have properties on your database you don't want to show outside your app. An example may be a createdDate, timestamp, or an encrypted value in the database. So your models may look something like

UserDatabaseModel (This is whats in your database)
    Id
    Name
    Email (stored encrypted in the database)
    CreatedDate
    Timestamp

UserViewModel (This is what you want to show your user)
    Id
    Name (shown not encrypted)
    Email

Your mapper would be responsible for maping a UserDatabaseModel to a UserViewModel and unencrypting the email (showing an encrypted email address to a user is pointless). And back the other way mapping a UserViewModel to a UserDatabaseModel and encrypting the email to store in your database. The Mapper will handle this behind the scenes so you don't have to worry about calling encrypt/decrypt all the time.

Unit of work and repository both work together. Unit of work controls a unit of work, so if for example updating a user actually updates 2 tables in your database, you would only want to save the details if both tables save. You wrap this in your unit of work (like a transaction in sql). Your repository is responsible for CRUDs or any other database actions. You would define the code that gets a user from the database or updates a user in your repository.

So updating the database from your code might look something like:

try{
    uow = new UnitOfWork //start a new database transaction

    repo = new UserRepository
    repo.UpdateUser //calls database
    repo.UpdateUserAddress //calls database

    uow.Save //commit the database transaction
} catch {
    uow.Rollback //something went wrong, rollback transaction
}

Data Mapper, Repository and Unit of Work patterns

1 Answers