====== Django ======

[[https://www.djangoproject.com/|Django]] is a high-level Python Web framework that encourages rapid development and clean, pragmatic design. 

===== View =====

===== Model =====

==== select_related ====

''select_related'' 用于解决ORM中的n+1问题（即在包含外键的对象中执行查询时，会查询n+1次）。

假设有数据库模型如下：

<code python models.py>
class User(models.Model):
  name = models.CharField(max_length=255)

class Article(models.Model):
  title = models.CharField(max_length=255)
  content = models.TextField()
  creator = models.ForeignField(User)
</code>

在一个列表中，**简单地** 查询文章列表及其作者，将会查询n+1次：

<code html index.html>
{% for article in articles %}
  <h2>{{ article.title }}</h2>
  <p>author: {{ article.creator.name }}</p>
{% endfor %}
</code>

然而使用 ''SQL'' 语言使用 ''inner join'' 只需要查询一次即可。

Django orm 使用 ''select_related'' 来解决这个问题：

<code python>
articles = Article.objects.all().select_related('creator')
</code>

==== Null && Blank ====

  * Null: It is database-related. Defines if a given database column will accept null values or not.
  * Blank: It is validation-related. It will be used during forms validation, when calling form.is_valid(). 

From [[https://simpleisbetterthancomplex.com/tips/2016/07/25/django-tip-8-blank-or-null.html|Django Tips #8 Blank or Null?]]


===== Template =====

===== Forms =====