Data Mapper Pattern || Repository || DAO || Julie Larmen 5 Data Access Patterns || EF
This is the Most Sophisticated of All Data Source Patterns and should be used for
Complex DDD domains.
Repositories are same as Data Mappers.
Data Mappers remove all the short comings of other 3 Data Source Patterns.
Other 3 Patterns are suitable for Simple CRUD domains having not Domain Logic.
Data Mappers are DDD/Onion Arch/Clean Arch oriented.
Data Mappers deals with Complex View based read operations.
Data Mappers deals with Complex Transaction oriented CUD operations involving many tables.
A layer of Mappers (473) that moves data between objects and a database while keeping them independent of each other and the mapper itself.
system with a complex domain model often benefits from a layer, such as the one provided by Data Mapper (165), that isolates domain objects from details of the database access code.
Active Record Pattern mixes everything in one class and it is row oriented. It puts Data, Getter Setters, Persistence Logic and Domain Logic in one class. this is BBOM. Table Data Gateway is an improvement on the ARP. It segregates the Persisence Logic from the Data and Domain Logic. TDG Pattern Figure does say anything about the Domain Logic but it could be there. Data Mapper Pattern is further improvement of TDG Pattern. It explicitly include the Domain Logic Methods. Secondly, it takes Domain Model obj (Entities) as input to the CUD methods
Class Person
{
Data Members:
Getter Setters
Domain Logic Methods
}
Class PersonMapper
{
CRUD (Person)
}
Look at the arrow in the figure above, Person Class is associated to the PersonMapper Class as the CU Operation refer the Person class.
Persistence Methods may be called as follows
PersonMapperObj.CRUDMethods(Person);
Table Data Gateway vs. Data Mapper Pattern
==========================================================
the Data Mapper will receive the Domain Model object(Entity) as param and will use it to implement the CRUD operations
Data Mapper takes the Entity and maps it to the underlying Table
Data Mapper is also Table Oriented, provide Table specific CRUD API
IT KNOWS ABOUT THE ENTITY
the Table Data Gateway will receives all the params(as primitives) for the methods and will not know anything about the Domain Model object(Entity).
TDG takes the Primitive Data Types and maps them to the underlying Table
TDG is also Table Oriented, provide Table specific CRUD API
IT DOES NOT KNOW ABOUT THE ENTITY
In the end both of them will act as mediator between the in-memory objects and the database.
Data Mapper (Entity) == Table Data Gateway (Primitive Data Types)
Repository vs. DataMapper Pattern
Repository/DataMapper/DAO: They are the same thing.
Data Mapper:
“A Data Mapper is a Data Access Layer that performs bidirectional transfers of data between DB and an in memory data representation (the domain layer).
The goal of the pattern is to keep the in memory representation and the DB independent of each other and the data mapper itself.
The layer is composed of one or more mappers (or Data Access Objects), performing the data transfer.
Mapper implementations vary in scope.
Generic mappers will handle many different domain entity types (Tables), dedicated mappers will handle one or a few (Tables).”
data mapper is meant to be a layer between the actual business domain of your application and the database that persists its data.
Java’s Hibernate and PHP’s Doctrine2 are the two prototypical data mapper facilitators
The Data Mapper is a layer of software that separates the in-memory objects from the database.
Its responsibility is to transfer data between the two and also to isolate them from each other.
With Data Mapper the in-memory objects needn't know even that there's a database present; they need no SQL interface code, and certainly no knowledge of the database schema. (The database schema is always ignorant of the objects that use it.)
Since it's a form of Mapper (473), Data Mapper itself is even unknown to the domain layer.
Data Mapper is perfect for representing non-trivial domain model. If your domain model is rather simple you might consider using Active Record pattern or Data Gateways.
Need for JOINs Data Mapper might sometimes (but not must) indicate that domain class is too complex.
In Domain-Driven Design domain classes (i.e. returned by Data Mapper) take form of entities or value objects.
A Data Mapper is a Data Access Layer that performs bidirectional transfer of data between a persistent data store (often a relational database) and an in-memory data representation (the domain layer). The goal of the pattern is to keep the in-memory representation and the persistent data store independent of each other and the data mapper itself. The layer is composed of one or more mappers (or Data Access Objects), performing the data transfer. Mapper implementations vary in scope. Generic mappers will handle many different domain entity types, dedicated mappers will handle one or a few.
implementations of the concept can be found in various frameworks for many programming environments.
Java/.NET[edit]
- MyBatis persistence framework
- Hibernate (NHibernate) persistence framework
- Use an ORM based on ActiveRecord if you do not have logic in your application.
- Use an ORM based on DataMapper for more complex models, especially if you need to abstract the domain objects from the database representation.
- When performance is needed, don’t try to optimize the ORM queries, just go back to pure SQL with a DataMapper.
Data Mapper
A layer of Mappers that moves data between objects and a database while keeping them independent of each other and the mapper itself.
Sort of "Man in the Middle".
!php
class BananaMapper
{
private $con;
public function __construct(Connection $con)
{
$this->con = $con;
}
public function persist(Banana $banana)
{
// code to save the banana
}
public function remove(Banana $banana)
{
// code to delete the banana
}
}
Data Mapper
Usage
!php
$banana = new Banana();
$banana->setName('Fantastic Banana');
$con = new Connection('...');
$mapper = new BananaMapper($con);
Persist = Save or Update
!php
$mapper->persist($banana);
Remove
!php
$mapper->remove($banana);
The Repository Pattern 
The Repository Pattern
A Repository mediates between the domain and data mapping layers, acting like an in-memory domain object collection.
!php
interface CustomerRepository
{
/**
* @return Customer
*/
public function find($customerId);
/**
* @return Customer[]
*/
public function findAll();
public function add(Customer $user);
public function remove(Customer $user);
}
The Repository Pattern
Client objects construct query specifications declaratively and submit them to Repository for satisfaction.
Objects can be added to and removed from the Repository, as they can from a simple collection of objects, and the mapping code encapsulated by the Repository will carry out the appropriate operations behind the scenes.
Conceptually, a Repository encapsulates the set of objects persisted in a data store and the operations performed over them, providing a more object-oriented view of the persistence layer.
Repository also supports the objective of achieving a clean separation and one-way dependency between the domain and data mapping layers.
# The Specification Pattern 
Data mapper pattern
A layer of Mappers that moves data between objects and a database while keeping them independent of each other and the mapper itself
How it works
A separate class works as a bridge between in-memory objects and the underlying (relational) database. The domain objects have no SQL interface code and no understanding of the database backend.
When to use it
When the database schema and object models differ and evolve independently. Objects can be designed and changed without consideration to the database (and only the mapper).
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