Yii2 framework backup

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Working with Databases

In this section, we will describe how to create a new page to display the country data fetched from from a database table country. To achieve this goal, you will configure a database connection, create an Active Record class, and then create an action and a view.

Through this tutorial, you will learn

How to configure a DB connection;
How to define an Active Record class;
How to query data using the Active Record class;
How to display data in a view in a paginated fashion.

Note that in order to finish this section, you should have basic knowledge and experience about databases. In particular, you should know how to create a database and how to execute SQL statements using a DB client tool.

Preparing a Database

To begin with, create a database named yii2basic from which you will fetch data in your application. You may create a SQLite, MySQL, PostgreSQL, MSSQL or Oracle database. For simplicity, we will use MySQL in the following description.

Create a table named country in the database and insert some sample data. You may run the following SQL statements.

CREATE TABLE `country` (
  `code` char(2) NOT NULL PRIMARY KEY,
  `name` char(52) NOT NULL,
  `population` int(11) NOT NULL DEFAULT '0'
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `Country` VALUES ('AU','Australia',18886000);
INSERT INTO `Country` VALUES ('BR','Brazil',170115000);
INSERT INTO `Country` VALUES ('CA','Canada',1147000);
INSERT INTO `Country` VALUES ('CN','China',1277558000);
INSERT INTO `Country` VALUES ('DE','Germany',82164700);
INSERT INTO `Country` VALUES ('FR','France',59225700);
INSERT INTO `Country` VALUES ('GB','United Kingdom',59623400);
INSERT INTO `Country` VALUES ('IN','India',1013662000);
INSERT INTO `Country` VALUES ('RU','Russia',146934000);
INSERT INTO `Country` VALUES ('US','United States',278357000);

To this end, you have a database named yii2basic, and within this database there is a country table with ten rows of data.

Configuring a DB Connection

Make sure you have installed the PDO PHP extension and the PDO driver for the database you are using (e.g. pdo_mysql for MySQL). This is a basic requirement if your application uses a relational database.

Open the file config/db.php and adjust the content based on your database information. By default, the file contains the following content:

<?php

return [
    'class' => 'yii\db\Connection',
    'dsn' => 'mysql:host=localhost;dbname=yii2basic',
    'username' => 'root',
    'password' => '',
    'charset' => 'utf8',
];

This is a typical file-based configuration. It specifies the parameters needed to create and initialize a yii\db\Connection instance through which you can make SQL queries against the underlying database.

The DB connection configured above can be accessed in the code via the expression Yii::$app->db.

Info: The config/db.php file will be included in the main application configuration config/web.php which specifies how the application instance should be initialized. For more information, please refer to the Configurations section.

Creating an Active Record

To represent and fetch the data in the country table, create an Active Record class named Country and save it in the file models/Country.php.

<?php

namespace app\models;

use yii\db\ActiveRecord;

class Country extends ActiveRecord
{
}

The Country class extends from yii\db\ActiveRecord. You do not need to write any code inside of it. Yii will guess the associated table name from the class name. In case this does not work, you may override the yii\db\ActiveRecord::tableName() method to explicitly specify the associated table name.

Using the Country class, you can manipulate the data in the country table easily. Below are some code snippets showing how you can make use of the Country class.

use app\models\Country;

// get all rows from the country table and order them by "name"
$countries = Country::find()->orderBy('name')->all();

// get the row whose primary key is "US"
$country = Country::findOne('US');

// displays "United States"
echo $country->name;

// modifies the country name to be "U.S.A." and save it to database
$country->name = 'U.S.A.';
$country->save();

Info: Active Record is a powerful way of accessing and manipulating database data in an object-oriented fashion. You may find more detailed information in the Active Record. Besides Active Record, you may also use a lower-level data accessing method called Data Access Objects.

Creating an Action

To expose the country data to end users, you need to create a new action. Instead of doing this in the site controller like you did in the previous sections, it makes more sense to create a new controller specifically for all actions about manipulating country data. Name this new controller as CountryController and create an index action in it, as shown in the following,

<?php

namespace app\controllers;

use yii\web\Controller;
use yii\data\Pagination;
use app\models\Country;

class CountryController extends Controller
{
    public function actionIndex()
    {
        $query = Country::find();

        $pagination = new Pagination([
            'defaultPageSize' => 5,
            'totalCount' => $query->count(),
        ]);

        $countries = $query->orderBy('name')
            ->offset($pagination->offset)
            ->limit($pagination->limit)
            ->all();

        return $this->render('index', [
            'countries' => $countries,
            'pagination' => $pagination,
        ]);
    }
}

Save the above code in the file controllers/CountryController.php.

The index action calls Country::find() to build a DB query and retrieve all data from the country table. To limit the number of countries returned in each request, the query is paginated with the help of a yii\data\Pagination object. The Pagination object serves for two purposes:

Sets the offset and limit clauses for the SQL statement represented by the query so that it only returns a single page of data (at most 5 rows in a page).
Being used in the view to display a pager consisting of a list of page buttons, as will be explained in the next subsection.

At the end, the index action renders a view named index and passes the country data as well as the pagination information to it.

Creating a View

Under the views directory, first create a sub-directory named country. This will used to hold all views rendered by the country controller. Within the views/country directory, create a file named index.php with the following content:

<?php
use yii\helpers\Html;
use yii\widgets\LinkPager;
?>
<h1>Countries</h1>
<ul>
<?php foreach ($countries as $country): ?>
    <li>
        <?= Html::encode("{$country->name} ({$country->code})") ?>:
        <?= $country->population ?>
    </li>
<?php endforeach; ?>
</ul>

<?= LinkPager::widget(['pagination' => $pagination]) ?>

The view consists of two parts. In the first part, the country data is traversed and rendered as an unordered HTML list. In the second part, a yii\widgets\LinkPager widget is rendered using the pagination information passed from the action. The LinkPager widget displays a list of page buttons. Clicking on any of them will refresh the country data in the corresponding page.

Trying it Out

To see how it works, use your browser to access the following URL:

http://hostname/index.php?r=country/index

You will see a page showing five countries. And below the countries, you will see a pager with four buttons. If you click on the button "2", you will see that the page displays another five countries in the database. Observe more carefully and you will find the URL in the browser changes to

http://hostname/index.php?r=country/index&page=2

Behind the scene, yii\data\Pagination is playing the magic.

Initially, yii\data\Pagination represents the first page, which sets the country query with the clause LIMIT 5 OFFSET 0. As a result, the first five countries will be fetched and displayed.
The yii\widgets\LinkPager widget renders the page buttons using the URLs created by yii\data\Pagination::createUrl(). The URLs will contain the query parameter page representing different page numbers.
If you click the page button "2", a new request for the route country/index will be triggered and handled. yii\data\Pagination reads the page query parameter and sets the current page number 2. The new country query will thus have the clause LIMIT 5 OFFSET 5 and return back the next five countries for display.

Summary

In this section, you have learned how to work with a database. You have also learned how to fetch and display data in pages with the help of yii\data\Pagination and yii\widgets\LinkPager.

In the next section, you will learn how to use the powerful code generation tool, called Gii, to help you rapidly implement some commonly required features, such as the Create-Read-Update-Delete (CRUD) operations about the data in a DB table. As a matter of fact, the code you have just written can all be automatically generated using this tool.

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