Object Relationship Mapping (ORM)

Introduction

• Relational Database: In a relational database, data is related together from table to table via special columns called primary and foreign keys.
• OOPs : We relate entities via references to each other through composition and aggregation.
• Object Relational Mapping (ORM) : Defines a mapping strategy to relate Java objects to relational tables.
• JPA : Defines relationships using fields and annotations in the entity classes. JPA mappings are established via annotations. 

Major Relationship Types Supported by JPA


1.  One-to-many

• Relate a row in a parent table to one or more rows in a child table.
• Relationships can either be defined as bi-directional or uni-directional.
• Two tables are related in a one-to-many (1—M) relationship if for every row in the first table, there can be zero, one, or many rows in the second table
• For every row in the second table there is exactly one row in the first table.
• The one-to-many relationship is also referred to as a parent-child or master-detail relationship.
• One-to-many relationships are the most commonly modeled relationship.
• eg: Student and Course
• One student can apply for one or more courses.

2.  One-to-one

• A single row in one table is related to a single row in another table.
• Two tables are related in a one-to-one (1—1) relationship if, for every row in the first table, there is at most one row in the second table.
• True one-to-one relationships seldom occur in the real world. 
• This type of relationship is often created to get around some limitation of the database management software rather than to model a real-world situation.
• eg: Student and Address
• One Student can have exactly one Address

3.  Many-to-Many

• Rows from each table are related to rows in another table.
• Two tables are related in a many-to-many (M—M) relationship when for every row in the first table, there can be many rows in the second table
• For every row in the second table, there can be many rows in the first table.
• Many-to-many relationships can't be directly modeled in relational database programs
• These types of relationships must be broken into multiple one-to-many relationships
• eg: Student and Teachers
• One Student is taught by many Teachers and one Teacher has many Students.

4.  Many-to-One

• A reference from a child entity back to its parent.
• eg: Course and Student
• Many Courses are applied by one Student.

5.  Inheritance

• JPA supports object-based inheritence and provides several physical models to map this onto a database.