djangoproject/first_django_app.md

# First Django App

Target: create a basic poll application, with 2 parts a **public site** and a **admin site**

* **public site**: let's pp view polls and vote
* **admin site**: let's you(admin) add/change/delete poll

## Part 1

1. Create web project
2. Create 1st app
3. Create 1st view
4. Add view & app into web url
### Creating a project

In the source directory, create project by

```
$ django-admin startproject <project_name>
```

Constraint on `<project_name>`:
* don't use names that conflict with django
  * e.g. `django`, `test` (test with built-in test file)

```
mysite/
    manage.py
    mysite/
        __init__.py
        settings.py
        urls.py
        asgi.py
        wsgi.py
```

Explain file structure:
* top-level `mysite/`: container of the project, can be renamed latter w/o affect Django framework
* `manage.py`: cmd-line utility let us to interact (admin) with Django projects.
* inner `mysite/` dir: actual Python package for the project. 
  * Importing any package from here: e.g. `mysite.urls`
* `mysite/__init__.py`: empty file, tell python start from here.
* `mysite/settings.py`: settings/config for this django project. 
* `mysite/urls.py`: URL declarations for this Django pjt; a "table of contents" of this Django website.
* `mysite/asgi.py`: An entry-point for ASGI-compatible webservers
* `mysite/wsgi.py`: Entry-point for WSGI-compatible webservers

### The development server

Start server steps:
1. cd into outer `mysite/` dir
2. Start server via: `python manage.py runserver`

Outcome:

* A development server is started. So no need to config a production server (e.g. Apache)
* Note: don't use it as production environment.

### Creating the Polls app

> Projects vs. apps
> 
> What’s the difference between a project and an app? An app is a Web application that does something – e.g., a Weblog system, a database of public records or a small poll app. A project is a collection of configuration and apps for a particular website. A project can contain multiple apps. An app can be in multiple projects.

Create a poll app along `mysite/`. For what?

```
$ python manage.py startapp polls
```

app hierarchy

```
polls/
    __init__.py
    admin.py
    apps.py
    migrations/
        __init__.py
    models.py
    tests.py
    views.py
```

### Write the first view

#### Step 1: Create a simple view of `polls` app

modify `polls/views.py` as

```python
from django.http import request
from django.shortcuts import render

def index(request):
    return HttpResponse("Hello, world. You're at the polls index")
```

* `def index(request):...` is the simplest view

#### Step 2: In `polls` app, map the URL to this view

After creating a view, map it to a URL so we can call it. Create `polls/urls.py`, and setup mapping `urlpatterns`

```python
from django.urls import path

from . import views

urlpatterns = [
    path('', views.index, name='index'),
]
```

#### Step 3: Point the root URLconf for polls

To point the root URLconf at `polls.urls` module:
1. In `mysite/urls.py`
   1. add import for `django.urls.inclue`
   2. insert a `include()` in `urlpatterns` list

i.e.

```python
from django.contrib import admin
from django.urls import include, path

urlpatterns = [
    path('polls/', include('polls.urls')),
    path('admin/', admin.site.urls),
]
```

##### django.urls.include()

> django.urls.include(): A function that takes a full Python import path to another URLconf module that should be “included” in this place.

* new imported `include()` function allows referencing other URLconfs.
* `'polls.urls'` is the package that we are using, hence `mysite` can connect to `polls` app

##### path()

Here `path()` function is passed **route** and **view**; two additional option available **kwargs**, and **name**

* `path()` argument **route**: a string that contains a URL pattern. When Django processing a request, it starts at the first pattern in `urlpatterns` and go down the list, comparing requested URL against each pattern
* `path()` argument **view**: when Django finds a matching pattern, it calls specified view function (a `HttpRequest` obj as the 1st argument, and others captured values as keyword argument)
* `path()` argument **kwargs**: passed in a dictionary to the target view
* `path()` argument **name**: naming URL, so we can refer to it elsewhere.

## Part 2

### Database setup

1. Modify `mysite/settings.py` for database binding
2. `python manage.py migrate` create database for each app

### Models Creation

在 Django 里写一个数据库驱动的 Web 应用的第一步: **定义模型** - 也就是数据库结构设计和附加的其它元数据 Meta Data

### Activate Models

改变模型需要这三步：

* 编辑 models.py 文件，改变模型
* 运行 python manage.py makemigrations 为模型的改变生成迁移文件。
* 运行 python manage.py migrate 来应用数据库迁移。

在这之前，确认polls已经在 INSTALLED_APPS的settings中。

详情如何使用 `manage.py` 可check [Django后台文档](https://docs.djangoproject.com/zh-hans/3.1/ref/django-admin/)

### Test API

进入 `python manage.py shell` 可以使用Django创建的各种API，如[数据库抽象API database API(建议细看)](https://docs.djangoproject.com/zh-hans/3.1/topics/db/queries/)

## Part 3

* In Django, web content & HTML come from Views models. Every view is a Python function/class. Django use customer requested URL to decide which View to generate.
* Django use 'URLconfs' to map btw URL and Views. Details of[pURL manager](https://docs.djangoproject.com/zh-hans/3.1/topics/http/urls/)
* URL's general form: `/newsarchive/<year>/<month>/`

### 3.1 Create more Views

> 当某人请求你网站的某一页面时——比如说， "/polls/34/" ，Django 将会载入 mysite.urls 模块，因为这在配置项 ROOT_URLCONF 中设置了。然后 Django 寻找名为 urlpatterns 变量并且按序匹配正则表达式。在找到匹配项 'polls/'，它切掉了匹配的文本（"polls/"），将剩余文本——"34/"，发送至 'polls.urls' URLconf 做进一步处理。在这里剩余文本匹配了 "<int:question_id>/"，使得我们 Django 以如下形式调用 polls.urls.detail():
> `detail(request=<HttpRequest object>, question_id=34)`

### 3.2 Create a true useful View

每个视图必须要做的只有两件事：返回一个包含被请求页面内容的 HttpResponse 对象，或者抛出一个异常，比如 Http404 。至于你还想干些什么，随便你。

你的视图可以从数据库里读取记录，可以使用一个模板引擎（比如 Django 自带的，或者其他第三方的），可以生成一个 PDF 文件，可以输出一个 XML，创建一个 ZIP 文件，你可以做任何你想做的事，使用任何你想用的 Python 库。

`django.shortcuts.render` 可以“载入模板，填充上下文，再返回由它生成的 HttpResponse 对象“

#### shortcut function: `render()`

「载入模板，填充上下文，再返回由它生成的 HttpResponse 对象」是一个非常常用的操作流程。于是 Django 提供了一个快捷函数 render

> The render() function takes the request object as its first argument, a template name as its second argument and a dictionary as its optional third argument. It returns an HttpResponse object of the given template rendered with the given context.

### 3.3 Throw 404 error

如果指定问题 ID 所对应的问题不存在，这个视图就会抛出一个 Http404 异常。

尝试用 get() 函数获取一个对象，如果不存在就抛出 Http404 错误也是一个普遍的流程。Django 也提供了一个快捷函数, 从而取代try/except语句

> 什么我们使用辅助函数 get_object_or_404() 而不是自己捕获 ObjectDoesNotExist 异常呢？还有，为什么模型 API 不直接抛出 ObjectDoesNotExist 而是抛出 Http404 呢？
> 因为这样做会增加模型层和视图层的耦合性。指导 Django 设计的最重要的思想之一就是要保证松散耦合。一些受控的耦合将会被包含在 django.shortcuts 模块中。

### 3.4 Using template system

[Templates in Django](https://docs.djangoproject.com/en/3.1/topics/templates/)

`polls/detail.html` 模板里正式的代码:

```html
<h1>{{ question.question_text }}</h1>
<ul>
{% for choice in question.choice_set.all %}
    <li>{{ choice.choice_text }}</li>
{% endfor %}
</ul>
```

模板系统统一使用点符号来访问变量的属性