CSC/ECE 517 Fall 2013/ch1 1w43 av

Introduction

Object-relational mapping (ORM) provides developers with a set of tools that ease management of the relationships between objects and relational databases, thus allowing applications to be easily extended to add data persistence. For Ruby, several object-relational mapping options are available. This wiki concentrates more on the comparison of ORMs and provides a high level overview of the top Ruby ORMs: ActiveRecord ,Sequel and DataMapper. It also includes a minor discussion on alternative frameworks that can be used either in place of or along with ORMs. These include the persistence framework iBATIS (more specifically the Ruby version RBatis).

Overview

Object-relational Mapping (ORM) frameworks unburden the designer of the complex translation between database and object space.

ORM features constitutes the following:

• It binds an object to its data in the database.
  • Class instance variables to database columns
  • Class instances to table rows
• It depicts associations, properties and behaviors related to tables and their fields.
  • Example, :has_many, :belongs_to associations in ActiveRecord
  • Inheritance cases are mapped to tables
• It manages the process of converting data between its database and object forms.
• It performs validation of data before it is persisted to table storage.
• It generates the SQL for a relational database to perform Create Read Update Delete operations in response to the changes made to the data objects within the application.

The diagram below depicts a simple mapping of an object to a database table….

Why ORM

A relational database has been the preferred mechanism for storing data requiring large storage space. An RDMBS provides users efficient create, read, update and delete operations. The efficiency of ORM technique of exchanging data between a relational db and an object oriented language surpasses the traditional techniques of data exchange.

The previously popular database products like structured query language had the disadvantage that they can operate over scalar values that have been organized in tabular format. This is where object relational mapping comes in the picture. It converts the object values into values suitable to be stored in the database. ORM helps in mapping between the logical business model and physical storage model . This gives the user the opportunity to model entities based on actual business requirements rather than the structure of the database. Object Relation mapping basically determines how objects and their relationships are persisted in permanent data storage.

A database ORM is an abstraction of a database. Few popular Ruby ORMs include Active Record, Data Mapper and Sequel. By enabling database rows to be used as objects, the program can more easily access and use the information in a way that is internally consistent and easy to understand. The ORM gives the programmer to manipulate data with the programming language, instead of having to manipulate each attribute as its data type as obtained by the database management system.

When applied to Ruby, implementations of ORM often leverage the language’s metaprogramming strengths to create intuitive application-specific methods and otherwise extend classes to support database functionality. With the addition of Rails, the ORM becomes much more important, as it is necessary to have an ORM to connect the models of the MVC (model-view-controller) stack used by Ruby on Rails with the application's database. Since the models are Ruby objects, the ORM allows modifications to the database to be done through changes to these models, independent of the type of database used.