Ruby on Rails 2015-06-15

Object-Relational Mapping

ORM libraries map database tables to classes. If a database has a table called products, the program will have a class named Product. Rows in the table correspond to objects of the class -- a particular product is represented as an object of class Product.

ActiveRecord is the ORM layer supplied with Rails. It closely follows the standard ORM model: tables map to classes, rows to objects, and columns to object attributes. By relying on convention and starting with sensible defaults, ActiveRecord minimizes the amount of configuration that developers perform.

To understand the ActiveRecord, here is the example:
In models/product.rb, a class Product is define:
class Product < ActiveRecord::Base
end

In db/migrate/xxx_create_products.rb, a class CreateProducts is define:
class CreateProducts < ActiveRecord::Migration
    def change
        create_table:products do |t|
        end
    end
end

By this way, Product has been mapped to the prodcuts table in the database. And the cool thing is that the Product calss that wrap the database table provides a set of class-level methods that perform table-level operations. To understand this, here is the example:
(1) find the product with a particular ID
product = Product.find(1)
(2) collect the objects whos name is 'dave'
product = Product.where(name: 'dave')
You don't need to care about how to manipulate the SQL query to get the data. Rails class wraps them for you and all you need to do is code like OO programming.