Active record pattern

In software engineering, the active record pattern is an architectural pattern found in software that stores in-memory object data in relational databases. It was named by Martin Fowler in his 2003 book Patterns of Enterprise Application Architecture. The interface of an object conforming to this pattern would include functions such as Insert, Update, and Delete, plus properties that correspond more or less directly to the columns in the underlying database table.

The active record pattern is an approach to accessing data in a database. A database table or view is wrapped into a class. Thus, an object instance is tied to a single row in the table. After creation of an object, a new row is added to the table upon save. Any object loaded gets its information from the database. When an object is updated the corresponding row in the table is also updated. The wrapper class implements accessor methods or properties for each column in the table or view.

This pattern is commonly used by object persistence tools, and in object-relational mapping (ORM). Typically, foreign key relationships will be exposed as an object instance of the appropriate type via a property.

Implementations

Implementations of the concept can be found in various frameworks for many programming environments. For example, if in a database there is a table parts with columns name (string type) and price (number type), and the Active Record pattern is implemented in the class Part, the pseudo-code

part = new Part()
part.name = "Sample part"
part.price = 123.45
part.save()

will create a new row in the parts table with the given values, and is roughly equivalent to the SQL command

INSERT INTO parts (name, price) VALUES ('Sample part', 123.45);

Conversely, the class can be used to query the database:

b = Part.find_first("name", "gearbox")

This will find a new Part object based on the first matching row from the parts table whose name column has the value "gearbox". The SQL command used might be similar to the following, depending on the SQL implementation details of the database:

SELECT * FROM parts WHERE name = 'gearbox' LIMIT 1; -- MySQL or PostgreSQL

ColdFusion

ColdFusion has an open source implementation of the Active Record pattern.

The ColdFusion on Wheels framework has an implementation of the Active Record pattern. It is open source and has the added advantage of requiring no complex configuration.

PHP

PHP ActiveRecord is one open-source library designed to fulfill the active record pattern.

Several open-source PHP frameworks also bundle their own ORM implementing the active record pattern. Most implementations support relationships, behaviors, validation, serialization and support for multiple data sources.

• Boiler, an MVC framework for PHP, contains a set of tools for auto-generation of active record models. The project, designed for data-centered projects, aims to automate as much of the development process as possible, using Apache Ant. Although a new addition to Open Source market, the project is already in use in many live applications, both commercially and open. The framework currently only supports MySQL though the developers have reported some commercial work in Postgres.
• Cygnite PHP Framework's default database layer implements Active Record pattern which closely resemble with Ruby on Rails.
• Laravel contains an ORM called 'Eloquent' which implements the active record pattern, closely resembling that of Ruby on Rails
• CakePHP's ORM implements the active record pattern, but as of version 2.x queries return arrays of data, with levels of related data as required. Version 3.0 uses objects.
• Lithium's ORM implements active record.
• Symfony's default database layer and ORM "Doctrine" does not implement active record but rather a data mapper approach.
• CodeIgniter has a query builder it calls "ActiveRecord", but which does not implement the Active Record pattern. Instead, it implements what the user guide refers to as a modified version of the pattern. The Active Record functionality in CodeIgniter can be achieved by using either CodeIgniter DataMapper library or CodeIgniter Gas ORM library.
• PHPixie's ORM implements active record.
• Yii's ORM also implements the active record pattern.
• Propel also implements the active record pattern.
• Paris is A lightweight Active Record implementation for PHP5, built on top of Idiorm.

Ruby

The Ruby library ActiveRecord implements ORM. It creates a persistable domain model from business objects and database tables, where logic and data are presented as a unified package. ActiveRecord adds inheritance and associations to the pattern above, solving two substantial limitations of that pattern. A set of macros acts as a domain language for the latter, and the Single Table Inheritance pattern is integrated for the former; thus, ActiveRecord increases the functionality of the active record pattern approach to database interaction. ActiveRecord is the default ‘model’ component of the model-view-controller web-application framework Ruby on Rails, and is also a stand-alone ORM package for other Ruby applications. In both forms, it was conceived of by David Heinemeier Hansson, and has been improved upon by a number of contributors.

Other, less popular ORMs have been released since ActiveRecord first took the stage. For example, DataMapper and Sequel show major improvements over the original ActiveRecord framework. As a response to their release and adoption by the Rails community, Ruby on Rails v3.0 is independent of an ORM system, so Rails users can easily plug in DataMapper or Sequel to use as their ORM of choice.

Java

The Java language implements the Active Record pattern via the ActiveJDBC library. ActiveJDBC is an implementation of Active Record design pattern inspired by Ruby on Rails ActiveRecord. ActiveJDBC is lightweight, fast, small and does not require any configuration.

ActiveJPA and jOOQ (for Java Object Oriented Querying) implements the Active record pattern, combining active records with source code generation and a querying DSL similar to SQL allowing for retrieving active records using complex SQL statements.

The Play framework is a Java web framework which implements the Active Record pattern, using ideas from Ruby on Rails.

JActiveRecord is yet another library providing easy ORM mapping for Java, inspired by Ruby on Rails ActiveRecord but more focused on Java's type-safety.

Other languages

There are several open source implementations of the Active Record pattern in other languages, including JavaScript (e.g., ActiveJS's Active Record), Perl (DBIx::Class), ActionScript, Python, Haxe (SPOD), C#, Objective-C and Scala.

Criticism

Testability

Due to the coupling of database interaction and application logic when using the active record pattern, unit testing an active record object without a database becomes difficult. The negative effects on testability in the active record pattern can be minimized by using mocking or dependency injection frameworks to substitute the real data tier with a simulated one.

See also

• Business object
• CRUD
• Data mapper pattern
• Object-relational mapping

External links

• Ruby implementation
• DBIx::Class Perl Implementation
• PHP implementation
• .NET implementation
• AIR Active Record project home
• troba Active record extension