CSC/ECE 517 Fall 2012/ch1b 1w63 dv

Active Records

Introduction

The Active Record pattern is a Design pattern in Software Engineering which deals with the approach to store and access data in a database. The interface of an object conforming to this pattern would contain functions to perform operations like Insert, Read, Update, and Delete. The Object will have properties that correspond to the columns in the underlying database table. This pattern is realised through ORM (Object-Relational Mapping) libraries in Programming languages.

ActiveRecord is a module for Ruby that can be used for ORM. Thus, it is obvious that ActiveRecord will form a part of the Model in an MVC application developed in Ruby. The rest of the article discusses ActiveRecord that is the Ruby module for implementing the Active Record pattern.

The ActiveRecord module insulates the developer from the need to use SQL in most cases. Internally, It will perform queries on the database which corresponds to the method invoked on the object. This module is compatible with most database systems (most used ones like MySQL, PostgreSQL and SQLite). Moreover, regardless of which database system the developer uses, the Active Record method format always remains the same.

Naming

The ActiveRecord module uses a convention for naming classes, tables and fields so that the amount of configuration needed to get the functionality working is minimal. There are naming conventions on file naming, class naming, table naming etc.

Reserved names and Attributes

Certain names are reserved and should not be used (even in the model as attributes). Some of them are listed below:

 lock_version.
 
 type - This is only used when you have single table inheritance and must contain a class name.
 
 id - Reserved for primary keys.
 
 table_name_count - Reserved for counter cache.
 
 position - Reserved for acts_as_list.
 
 parent_id - Reserved for acts_as_tree.
 
 lft - Reserved for acts_as_nested_set.
 
 rgt - Reserved for acts_as_nested_set.
 
 quote - Method in ActiveRecord::Base which is used to quote SQL.
 
 template.
 

Class Naming

ActiveRecord classes are named in singular form. e.g User

Table Naming

Tables for ActiveRecord objects are named in plural form by default. e.g. Users This naming convention can be circumvented by using below:

a) Set use_pluralization In the environment.rb file we can specify

  ActiveRecord::Base.use_pluralization = false. 

This will disable pluralization for all ActiveRecord objects.

b.) Use set_table_name You can call set_table_name to specify a custom table name for a particular model. For example:

 class User < ActiveRecord::Base
   set_table_name 'user'
 end

3 CRUD

   3.1 Create
           <Use relevant code snippets>
   3.2 Read
  3.3 Update
  3.4 Delete

Connecting to the Database

Hidden underneath ActiveRecord class is a useful low-level object called the ActiveRecord connection adapter. It wraps and abstracts away the underlying database-specific driver, and provides a common interface for database tasks such as creating and destroying databases, creating and modifying tables, inserting, updating, and deleting data, running queries, and managing transactions. Normally, the connection adapter is used internally by ActiveRecord, but you can access it yourself if you want to talk to the database directly without using ActiveRecord “models.”

To obtain a connection adapter object, simply call the connection method on your ActiveRecord class or any ActiveRecord object:

 connection = User.connection
 obj = User.find(1)
 connection = obj.connection

In most Rails applications, you talk to just one database, as defined in your database.yml file. For such an application, every ActiveRecord class, including ActiveRecord::Base, will give you the same connection. So in those cases, it doesn’t matter which class you use. However, if your Rails application connects to a secondary database for some ActiveRecord classes (using the establish_connection method) then those classes will yield a connection object pointing at the secondary database. In those cases, you will need to pay attention to which database you need to talk to, and ask for a connection from the right class.

Each connection adapter object represents a single connection to a database. Rails generally opens several connections at once and manages them in a connection pool. When a task needs a database connection, it checks one out of the pool; when it finishes, it checks the connection back in so that the next task can use it. Connections can run only one SQL statement at a time, so generally one connection is opened per thread.