Active Record Pattern

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In computer science, the active record pattern is a design pattern frequently found in software that stores its data in relational databases. It was named by Martin Fowler in his book Patterns of Enterprise Application Architecture.

Active record 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. Typically foreign key relationships will be exposed as an object instance of the appropriate type via a property.

Implementations of Active Record 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 following 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_part("name", "gearbox")

This will create 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 would be

SELECT * FROM parts WHERE name = 'gearbox' LIMIT 1;

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