Object-relational mapper FRO DJANGO

Define your data models entirely in Python. You get a rich, dynamic database-access API for free — but you can still write SQL if needed.

for example:

class Band(models.Model):
    """A model of a rock band."""
    name = models.CharField(max_length=200)
    can_rock = models.BooleanField(default=True)


class Member(models.Model):
    """A model of a rock band member."""
    name = models.CharField("Member's name", max_length=200)
    instrument = models.CharField(choices=(
            ('g', "Guitar"),
            ('b', "Bass"),
            ('d', "Drums"),
        ),
        max_length=1
    )
    band = models.ForeignKey("Band")

Models¶

A model is the single, definitive source of information about your data. It contains the essential fields and behaviors of the data you’re storing. Generally, each model maps to a single database table.

The basics:

• Each model is a Python class that subclasses  django.db.models.Model .
• Each attribute of the model represents a database field.
• With all of this, Django gives you an automatically-generated database-access API; see Making queries.

from django.db import models

class Person(models.Model):
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30)

　　The above Person model would create a database table like this: 上面的将会相当于用下面的SQL语句在数据库中创建表格

CREATE TABLE myapp_person (
    "id" serial NOT NULL PRIMARY KEY,
    "first_name" varchar(30) NOT NULL,
    "last_name" varchar(30) NOT NULL
);

　　

使用模型¶

一旦定义了模型，就需要告诉Django你将使用 这些模型。通过编辑您的设置文件并更改INSTALLED_APPS设置以添加包含您的 设置的模块的名称来执行此操作models.py。

例如，如果应用程序的模型位于模块中 myapp.models（脚本为应用程序创建的包结构）， 则应部分阅读：manage.pystartappINSTALLED_APPS

INSTALLED_APPS = [
    #...
    'myapp',
    #...
]

　　When you add new apps to INSTALLED_APPS, be sure to run manage.py migrate, optionally making migrations for them first with manage.py makemigrations.

Fields¶

The most important part of a model – and the only required part of a model – is the list of database fields it defines. Fields are specified by class attributes. Be careful not to choose field names that conflict with the models API like clean, save, ordelete.

Example:

from django.db import models

class Musician(models.Model):
    first_name = models.CharField(max_length=50)
    last_name = models.CharField(max_length=50)
    instrument = models.CharField(max_length=100)

class Album(models.Model):
    artist = models.ForeignKey(Musician, on_delete=models.CASCADE)
    name = models.CharField(max_length=100)
    release_date = models.DateField()
    num_stars = models.IntegerField()

choices

An iterable (e.g., a list or tuple) of 2-tuples to use as choices for this field. If this is given, the default form widget will be a select box instead of the standard text field and will limit choices to the choices given.

A choices list looks like this:

YEAR_IN_SCHOOL_CHOICES = (
    ('FR', 'Freshman'),
    ('SO', 'Sophomore'),
    ('JR', 'Junior'),
    ('SR', 'Senior'),
    ('GR', 'Graduate'),
)

　　

The first element in each tuple is the value that will be stored in the database. The second element is displayed by the field’s form widget.

Given a model instance, the display value for a field with choices can be accessed using the get_FOO_display()method. For example:

from django.db import models

class Person(models.Model):
    SHIRT_SIZES = (
        ('S', 'Small'),
        ('M', 'Medium'),
        ('L', 'Large'),
    )
    name = models.CharField(max_length=60)
    shirt_size = models.CharField(max_length=1, choices=SHIRT_SIZES)

　　

>>> p = Person(name="Fred Flintstone", shirt_size="L")
>>> p.save()
>>> p.shirt_size
'L'
>>> p.get_shirt_size_display()
'Large'