Page 266 - Asterisk™: The Future of Telephony

class Customer < ActiveRecord::Base
belongs_to :group
validates_presence_of :name, :phone_number
validates_uniqueness_of :phone_number
validates_associated :group
def total_bill
self.group.hourly_rate * self.usage_this_month / 1.hour
end
end
class Group < ActiveRecord::Base
has_many :customers
validates_presence_of :description, :hourly_rate
end
From just this small amount of information, ActiveRecord can make a lot of logical
inferences. When these classes interpret, ActiveRecord assumes the table names to be
customers and groups respectively by lowercasing the classes’ names and making them
plural. If this convention is not desired, the author can easily override it. Additionally,
at interpretation time, ActiveRecord actually peeks into the database’s columns and
makes available many new dynamically created methods.
The belongs_to and has_many methods in this example define relationships between
Customers and Groups. Notice again how ActiveRecord uses pluralization to make the
code more expressive in the has_many :customers line. From this example, we also see
several validations—policies that ActiveRecord will enforce. When creating a new
Customer we must provide a name and phone_number at the bare minimum. No two phone
numbers can conflict. Every Customer must have a Group. Every Group must have a
description and hourly_rate. These help both the developer and the database stay on
track.
Also, notice the total_bill method in the Customer class. On any Customer object we
extract from the database, we can call this method, which multiplies the hourly_rate
value of the group to which the Customer belongs by the Customer’s own phone usage
this month (in seconds).
Here are a few examples that may clarify the usefulness of having Ruby objects abstract
database logic:
everyone = Customer.find :all
jay = Customer.find_by_name "Jay Phillips"
jay.phone_number # Performs a SELECT statement
jay.total_bill # Performs arithmetic on several SELECT statements
jay.group.customers.average :usage_this_month
jay.group.destroy
jay.group = Group.create :description => "New cool group!",
:hourly_rate => 1.23
jay.save

238 | Chapter 10: Asterisk Manager Interface (AMI) and Adhearsion