Этот раздел расскажет о том, как создать новую страницу, отображающую данные по странам, полученные из таблицы `countries` базы данных. Для достижения этой цели вам будет необходимо настроить подключение к базе данных, создать класс [Active Record](db-active-record.md), определить [action](structure-controllers.md), и создать [view](structure-views.md).
Изучив эту часть, вы научитесь:
* Настраивать подключение к БД
* Определять класс Active Record
* Запрашивать данные, используя класс Active Record
* Отображать данные в виде с использованием пагинации
Обратите внимание, чтобы усвоить этот раздел, вы должны иметь базовые знания и навыки использования баз данных.
В частности, вы должны знать, как создать базу данных, и как выполнять SQL запросы, используя клиентские инструменты для работы с БД.
Подготавливаем базу данных <a name="preparing-database"></a>
----------------------
Для начала, создайте базу данных под названием `yii2basic`, из которой вы будете получать данные в вашем приложении.
Вы можете создать базу данных SQLite, MySQL, PostgreSQL, MSSQL или Oracle, так как Yii имеет встроенную поддержку для многих баз данных. Для простоты, в дальнейшем описании будет подразумеваться MySQL.
После этого создайте в базе данных таблицу `country`, и добавьте в неё немного демонстрационных данных. Вы можете запустить следующую SQL инструкцию, чтобы сделать это:
На данный момент у вас есть база данных под названием `yii2basic`, и внутри неё таблица `country` с тремя столбцами, содержащими десять строк данных.
Настраиваем подключение к БД <a name="configuring-db-connection"></a>
---------------------------
Before proceeding, make sure you have installed both the [PDO](http://www.php.net/manual/en/book.pdo.php) 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.
With those installed, open the file `config/db.php` and change the parameters to be correct for your database. By default,
the file contains the following:
```php
<?php
return[
'class'=>'yii\db\Connection',
'dsn'=>'mysql:host=localhost;dbname=yii2basic',
'username'=>'root',
'password'=>'',
'charset'=>'utf8',
];
```
The `config/db.php` file is a typical file-based [configuration](concept-configurations.md) tool. This particular configuration file 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 application code via the expression `Yii::$app->db`.
> Info: The `config/db.php` file will be included by the main application configuration `config/web.php`,
which specifies how the [application](structure-applications.md) instance should be initialized.
For more information, please refer to the [Configurations](concept-configurations.md) section.
Creating an Active Record <a name="creating-active-record"></a>
-------------------------
To represent and fetch the data in the `country` table, create an [Active Record](db-active-record.md)-derived
class named `Country`, and save it in the file `models/Country.php`.
```php
<?php
namespaceapp\models;
useyii\db\ActiveRecord;
classCountryextendsActiveRecord
{
}
```
The `Country` class extends from [[yii\db\ActiveRecord]]. You do not need to write any code inside of it! With just the above code,
Yii will guess the associated table name from the class name.
> Info: If no direct match can be made from the class name to the table name, you can
override the [[yii\db\ActiveRecord::tableName()]] method to explicitly specify the associated table name.
Using the `Country` class, you can easily manipulate data in the `country` table, as shown in these snippets:
```php
useapp\models\Country;
// get all rows from the country table and order them by "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 to access and manipulate database data in an object-oriented fashion.
You may find more detailed information in the [Active Record](db-active-record.md) section. Alternatively, you may also interact with a database using a lower-level data accessing method called [Data Access Objects](db-dao.md).
Creating an Action <a name="creating-action"></a>
------------------
To expose the country data to end users, you need to create a new action. Instead of placing the new action in the `site`
controller, like you did in the previous sections, it makes more sense to create a new controller specifically
for all actions related to the country data. Name this new controller `CountryController`, and create
an `index` action in it, as shown in the following.
```php
<?php
namespaceapp\controllers;
useyii\web\Controller;
useyii\data\Pagination;
useapp\models\Country;
classCountryControllerextendsController
{
publicfunctionactionIndex()
{
$query=Country::find();
$pagination=newPagination([
'defaultPageSize'=>5,
'totalCount'=>$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()`. This Active Record method builds a DB query and retrieves all of the 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 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 a time (at most 5 rows in a page).
* It's 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 of the code, the `index` action renders a view named `index`, and passes the country data as well as the pagination
information to it.
Creating a View <a name="creating-view"></a>
---------------
Under the `views` directory, first create a sub-directory named `country`. This folder will be used to hold all the
views rendered by the `country` controller. Within the `views/country` directory, create a file named `index.php`
The view has two sections relative to displaying the country data. In the first part, the provided 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 <a name="trying-it-out"></a>
-------------
To see how all of the above code works, use your browser to access the following URL:
```
http://hostname/index.php?r=country/index
```
At first, you will see a page showing five countries. Below the countries, you will see a pager with four buttons.
If you click on the button "2", you will see the page display another five countries in the database: the second page of records.
Observe more carefully and you will find that the URL in the browser also changes to
```
http://hostname/index.php?r=country/index&page=2
```
Behind the scenes, [[yii\data\Pagination|Pagination]] is providing all of the necessary functionality to paginate a data set:
* Initially, [[yii\data\Pagination|Pagination]] represents the first page, which reflects the country SELECT query
with the clause `LIMIT 5 OFFSET 0`. As a result, the first five countries will be fetched and displayed.
* The [[yii\widgets\LinkPager|LinkPager]] widget renders the page buttons using the URLs
created by [[yii\data\Pagination::createUrl()|Pagination]]. The URLs will contain the query parameter `page`, which
represents the 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|Pagination]] reads the `page` query parameter from the URL and sets the current page number to 2.
The new country query will thus have the clause `LIMIT 5 OFFSET 5` and return the next five countries
for display.
Summary <a name="summary"></a>
-------
In this section, you learned how to work with a database. You 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](tool-gii.md),
to help you rapidly implement some commonly required features, such as the Create-Read-Update-Delete (CRUD)
operations for working with the data in a database table. As a matter of fact, the code you have just written can all
be automatically generated in Yii using the Gii tool.
