Need for ORM

Requirement: How to store Objects into Database tables.

 Many applications are required to persistent the data into database. In Java application development, every data is represented in the form Objects. But most of the organizations are used to store data into RDBMS. But in RDBMS data is represented in the form of tables. Here the problem is how Object data is stored into Database Relations (i.e. in Tabular Format).  It leads to create 5 types of mismatch problems they are listed below.

1. Granularity

Sometimes you will have an object model which has more classes than the number of corresponding tables in the database (we says the object model is more granular than the relational model).  Take for example the notion of an Address.

For example you have a class called User and Address. But User has both billing and home address filed. I design my data base model as more granular. I explain in the following tables.

User

+String name +Address home +Address  billing

Address

+String street +String state +String country

       

NAME	HOME_STREET	HOME_STATE	HOME_COUNTRY	BILL_STREET	BILL_STATE	BILL_COUNTRY

Note: Granularity model which describes more classes than tables.

2. Subtypes (inheritance)

Inheritance is a natural paradigm in object-oriented programming languages. However, RDBMS do not define anything similar on the whole (yes some databases do have subtype support but it is completely non-standardized).

3. Identity

A RDBMS defines exactly one notion of 'sameness': the primary key.  Java, however, defines both object identity (a==b) and object equality (a. equals (b)).

4. Associations

Normally the associations are represented in RDBMS in the form of relations. But in Java language associations are represented by using the Object references.   In database the relations are allows to navigable in bidirectional. But in Java Object references are directional, the association from one object from another.

In relational world, an association is represented as a foreign key.

5. Data navigation

  It is important because, the performance of the application will be depends. For each application required to retrieve data from database by sending queries to database. If the number of communications increases, then automatically it decreases the application performance. We discus about this problem in up coming chapters.

 ORM: (Object Relational Mapping)

The Object Relational Mapping is a technique which is used to map Objects data in to Database tables. ORM is a technique which transforms data from one format to other.

ORM has the following support:

1.             It has API to perform basic CRUD operations.

2.             A language API supports to specify queries and that refer to properties and classes.

3.             A facility for specifying mapping Meta data.

4.             Supports to interact with transactional objects.

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