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 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 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.
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.
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. It largely simplifies object-record mapping by assuming homogeneous identification method (i.e. Surrogate key) by virtue of convention over configuration. 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 ORMs have been released since ActiveRecord first appeared. For example, DataMapper and Sequel. These ORMs include improvements over the original ActiveRecord framework.[neutrality is disputed] As a response to their release and adoption by the Rails community, Ruby on Rails v3.0 became independent of an ORM system, so that Rails users can instead use DataMapper or Sequel as their ORM.
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.
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.
Single responsibility principle and separation of concerns
Another critique of the active record pattern is that, also due to the strong coupling of database interaction and application logic, an active record object does not follow the single responsibility principle and separation of concerns as opposed to multitier architecture which properly addresses these practices. Because of this, the active record pattern is best and most often employed in simple applications that are all forms-over-data with CRUD functionality, or only as one part of an architecture. Typically that part is data access and why several ORMs implement the active record pattern.
Record based patterns work poorly in distributed systems especially where concurrency is impossible ( eg offline ). i.e. 2 updates both may have 1 field that is correct but only one of the 2 records can win.[clarification needed]
- Fowler, Martin (2003). Patterns of enterprise application architecture. Addison-Wesley. ISBN 978-0-321-12742-6.
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