Monday, 6 August 2012

Introduction - Yii


Yii is a high-performance, component-based application development framework written in PHP. It helps ease the complexity of building large-scale applications. It enables maximum reusability in web programming, and can significantly accelerate the development process. It does so by allowing the developer to build on top of already well-written, well-tested, and production-ready code. It prevents you from having to rewrite core functionality that is common across many of today's web-based applications, allowing you to concentrate on the business rules and logic specific to the unique application being built.

MVC architecture:

Yii is an MVC framework and it provides an explicit folder structure for each piece of model, view, and controller code. Before we start building our first Yii application, we need to define a few key terms, and look at how Yii implements and enforces this MVC architecture.

The model:

Typically in an MVC architecture, the model is responsible for maintaining state.Thus, it should encapsulate the business rules that apply to the data that defines this state. A model in Yii is any instance of the framework class CModel or its child class. A model class typically comprises data attributes that can have separate labels (something user-friendly for the purpose of display), and can be validated against a set of rules defined in the model. 

The data that makes up the attributes in the model class could come from a row of a database table or from the fields in a user input form.Yii implements two kinds of models: 
  1. The form model (CFormModel class) and
  2. The active record model (CActiveRecord class). 
They both extend from the same base class CModel.  

CFormModel represents a data model that collects inputs in HTML form. It encapsulates all the logic for form field validation and any other business logic that may need to be applied to the form field data. It can then store this data in memory, or with the help of an active record model, store data in a database.
Active Record (AR) is a design pattern used to abstract database access in an object-oriented fashion. Each AR object in Yii is an instance of CActiveRecord or its child class that wraps a single row in a database table or view, encapsulates all the logic and details around database access, and houses much of the business logic that is required to be applied to that data. The data field values for each column in the table row are represented as properties of the AR object. AR is described in more detail a little later.

The view:

Typically, the view is responsible for rendering the user interface, based on the data in the model. A view in Yii is a PHP script that contains user interface related elements, often built using HTML, but can also contain PHP statements. Usually any PHP statements within the view are very simple conditional or looping statements,or refer to other Yii UI-related elements such as HTML helper class methods or prebuilt widgets. More sophisticated logic should be separated from the view and placed appropriately in either the model (if dealing directly with the data), or in the controller for a more general business logic.

The controller:

The controller is our main director of a routed request and is responsible for taking user input,interacting with the model, and instructing the view to update and display appropriately. A controller in Yii is an instance of CController or its child. When a controller runs, it performs the requested action, which then interacts with needed models and renders an appropriate view. An action, in its simplest form, is a controller class method whose name starts with the word action.

Object-relational mapping and Active Record


For the most part, the web applications we build house their data in a relational database. However, web applications need the data that is held in the persistent database storage mapped to in-memory class properties that define the domain objects. Object-relational mapping (ORM) libraries provide this mapping of database tables to domain object classes.

Much of the code that deals with ORM is about describing how fields in the database correspond to fields in our objects, which is tedious and repetitive to write. Luckily, Yii comes to our rescue to save us from this repetition and tedium by providing the ORM layer in the form of the AR pattern.

Active Record

As was previously mentioned, AR is a design pattern used to abstract database access in an object-oriented fashion. It maps tables to classes, rows to objects and columns to object attributes. In other words, each instance of an active record class represents a single row in a database table. However an AR class is more than just a set of attributes that map to columns in a database table; it also houses the needed business logic behavior to be applied to that data. The end result is a class that defines everything about how it should be written to and read from the database.

By relying on convention and sticking with reasonable defaults, Yii's implementation of AR will save the developer a ton of time normally spent in configuration or in writing tedious and repetitive SQL statements required to create, read, update and delete data. It also allows the developer to access data stored in the database in a much more object-oriented way.

To illustrate this, here is some example code that uses AR to operate on a specific blog posting whose internal id, which is also used as the table's Primary Key, is 99. It first retrieves the posting by Primary Key, it then changes the title, and then updates the database to save the changes:

$post->title='Some new title';

Active Record completely relieves us of the tedium of having to write any SQL or otherwise deal with the underlying database.