Er To Relational Mapping Pdf Download
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The Relational part is a Relational Database Manager System ( A database that is ) there are other types of databases but the most popular is relational ( you know tables, columns, pk fk etc eg Oracle MySQL, MS-SQL )
Like all acronyms it's ambiguous, but I assume they mean object-relational mapper -- a way to cover your eyes and make believe there's no SQL underneath, but rather it's all objects;-). Not really true, of course, and not without problems -- the always colorful Jeff Atwood has described ORM as the Vietnam of CS;-). But, if you know little or no SQL, and have a pretty simple / small-scale problem, they can save you time!-)
Object Model is concerned with the following three conceptsData AbstractionEncapsulationInheritanceThe relational model used the basic concept of a relation or table.Object-relational mapping (OR mapping) products integrate object programming language capabilities with relational databases.
Object-oriented databases tend to be used in complex, niche applications. One of the arguments against using an OODBMS is that it may not be able to execute ad-hoc, application-independent queries.[citation needed] For this reason, many programmers find themselves more at home with an object-SQL mapping system, even though most object-oriented databases are able to process SQL queries to a limited extent. Other OODBMS provide replication to SQL databases, as a means of addressing the need for ad-hoc queries, while preserving well-known query patterns.[citation needed]
When interacting with a database using OOP languages, you'll have to perform different operations like creating, reading, updating, and deleting (CRUD) data from a database. By design, you use SQL for performing these operations in relational databases.
I had an opportunity to meet Ted Neward at TechEd this year. Ted, among other things, famously coined the phrase "Object-Relational mapping is the Vietnam of our industry" in late 2004.
It's a scary analogy, but an apt one. I've seen developers struggle for years with the huge mismatch between relational database models and traditional object models. And all the solutions they come up with seem to make the problem worse. I agree with Ted completely; there is no good solution to the object/relational mapping problem. There are solutions, sure, but they all involve serious, painful tradeoffs. And the worst part is that you can't usually see the consequences of these tradeoffs until much later in the development cycle.
Personally, I think the only workable solution to the ORM problem is to pick one or the other: either abandon relational databases, or abandon objects. If you take the O or the R out of the equation, you no longer have a mapping problem.
Object-relational mapping (ORM) is a way to align programming code with database structures. ORM uses metadata descriptors to create a layer between the programming language and a relational database. It thus connects object-oriented program (OOP) code with the database and simplifies the interaction between relational databases and OOP languages.
Developers can also perform various data creating, reading, updating and deleting (CRUD) operations in relational databases without using SQL. This capability is particularly useful for developers who either don't know SQL or don't want to waste time writing SQL code. With ORM, they don't have to understand and write SQL or rely on SQL query builders to add an abstraction layer to the SQL code.
This information helps developers understand the underlying database structure. When the application makes changes to the data object, the relational database will insert, update, create or delete data in response to these changes. This happens because the ORM converts data between tables and generates the SQL code needed by the database to respond to application changes and to manage data activities.
ORM manages the mapping details between a set of objects and underlying relational databases, XML repositories or other data sources and sinks. It simultaneously hides the often changing details of related interfaces from developers and the code they create. In many cases, ORM changes can incorporate new technology and capabilities without requiring changes to the code for related applications.
Incorrect mapping between data tables and objects can also create application problems that may be difficult to recognize and can affect overall performance. Finally, if the ORM layer is poorly written, it can become difficult to improve data schemas and to manage database migrations.
A relational mapping transforms any object data member type to a corresponding relational database (SQL) data source representation in any supported relational database. Relational mappings let you map an object model into a relational data model.
Relational mappings transform object data members to relational database fields. Use them to map simple data types including primitives (such as int), JDK classes (such as String), and large object (LOB) values. You can also use them to transform object data members that reference other domain objects by way of association where data source representations require object identity maintenance (such as sequencing and back references) and possess various types of multiplicity and navigability. The appropriate mapping class is chosen primarily by the cardinality of the relationship.
Do not confuse relational mappings with object-relational data type mappings (see Chapter 40, "Introduction to Object-Relational Data Type Mappings"). An object-relational data type mapping transforms certain object data member types to structured data source representations optimized for storage in specialized object-relational data type databases, such as Oracle Database. Object-relational data type mappings let you map an object model into an object-relational data type data model. In general, you can use relational mappings with any supported relational database. You can only use object-relational data type mappings with specialized object-relational data type databases optimized to support object-relational data type data source representations.
The direction of a relationship may be either unidirectional or bidirectional. In a unidirectional relationship, only one entity bean has a relationship field that refers to the other. All TopLink relational mappings are unidirectional, from the class being described (the source class) to the class with which it is associated (the target class). The target class does not have a reference to the source class in a unidirectional relationship.
In a bidirectional relationship, each entity bean has a relationship field that refers to the other bean. Through the relationship field, an entity bean's code can access its related object. To implement a bidirectional relationship (classes that reference each other), use two unidirectional mappings with the sources and targets reversed.
If the attribute type is comparable to a database type but requires conversion, the information can be stored directly by using a direct-to-field mapping (see Section 27.3, "Direct-to-Field Mapping") and an appropriate Converter instance.
In the previous release, TopLink provided subclasses of DirectToFieldMapping for object type direct mappings, serialized object direct mappings, and type conversion direct mappings. In this release, these subclasses are deprecated. In their place, Oracle recommends that you use the DirectToFieldMapping method setConverter and the corresponding Converter instance. Table 27-2 summarizes these changes.
There are some special considerations when using one-to-one mappings (see Section 27.5.1, "One-to-One Mappings and EJB 2.n CMP"), one-to-many mappings (see Section 27.7.1, "One-to-Many Mappings and EJB 2.n CMP"), and many-to-many mappings (see Section 27.8.1, "Many-to-Many Mappings and EJB 2.n CMP").
Use direct-to-field mappings to map primitive object attributes, or non persistent regular objects, such as the JDK classes. For example, use a direct-to-field mapping to store a String attribute in a VARCHAR field.
Figure 27-1 illustrates a direct-to-field mapping between the Java attribute city and the relational database column CITY. Similarly, direct-to-field mappings could be defined from country to COUNTRY, id to ADDRESS_ID, established to EST_DATE, and province to PROVINCE.
If you plan to use direct-to-XMLType mappings in TopLink Workbench and the TopLink runtime, you must include the Oracle Database xdb.jar file in the TopLink Workbench classpath (see Section 5.2, "Configuring the TopLink Workbench Environment").
One-to-one mappings represent simple pointer references between two Java objects. In Java, a single pointer stored in an attribute represents the mapping between the source and target objects. Relational database tables implement these mappings using foreign keys.
Figure 27-2 illustrates a one-to-one relationship from the address attribute of an Employee object to an Address object. To store this relationship in the database, create a one-to-one mapping between the address attribute and the Address class. This mapping stores the id of the Address instance in the EMPLOYEE table when the Employee instance is written. It also links the Employee instance to the Address instance when the Employee is read from the database. Because an Address does not have any references to the Employee, it does not have to provide a mapping to Employee.
You can also implement a one-to-one mapping where the target table contains a foreign key reference to the source table. In Figure 27-2, the database design would change such that the ADDRESS row would contain the EMP_ID to identify the Employee to which it belonged. In this case, the target must also have a relationship mapping to the source.
The update, insert and delete operations, which are normally done for the target before the source for privately owned one-to-one relationships, are performed in the opposite order when the target owns the foreign key. Target foreign keys normally occur in bidirectional one-to-one mappings (see Section 27.2.1, "Directionality"), because one side has a foreign key and the other shares the same foreign key in the other's table.
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