Active Record

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Apps generally contain three types of things: The page the user sees (the view), the saved information (the model), and the logic that delivers the right information to the right user (the controller).

Active Record is a part of Rails that standardizes and simplifies many common tasks involved in working with a part of the model layer--databases.

In order to use the standardized methods that Active Record provides for working with databases, you'll have to make sure your model classes subclass a class defined in the Active Record library named "Base" (since it's the base for your model).

By default, Rails will make sure your model classes get the subclassing right if you use the rails generate model command to create your model. For example, once you have a new Rails app, you can enter the command line, cd into the Rails root, and type:

rails generate model User username:string password:string

In this example we've created a model for a single User, who has a username and a password (which are both strings). With this one magic command, Rails creates a file named user.rb in the app/models folder of the Rails root (as well as a few other files we'll explore briefly). By default, this is the generated file:

# The file inherits from ActiveRecord's base class
class User < ActiveRecord::Base				  
	attr_accessible :username, :password

By subclassing Base, the User class inherits all the methods defined in the Base class in the Active Record gem. These methods act as a bridge between instances of the model class and the database table these instances relate to (in this example, the users table--note the pluralization of the given input, User).

What is the Base class?

The Base class in Active Record is an Object-Relational Mapper, meaning it abstracts the details of working with a relational database. Instead of writing SQL statements (or statements in a similar Data Definition Language, you can write statements in Ruby:

joebiden ={username: "joebiden", password:"VP4life"})

These Ruby statements, which use Ruby methods (new and save) and Ruby data types (hashes, strings, variables), translate themselves into the SQL statement: INSERT INTO users(primary key, username, password) VALUES (2, "joebiden", "VP4life);

Which inserts the data into the database. A key difference we can see already is that Active Record handles the messy business of managing primary keys for us, whereas in SQL we need to ensure we're using a unique integer as a primary key.

Active Record is one of two libraries (gems) that ship with Rails that are used to access the model layer (the other being Active Model).