《Django 5 By Example》阅读笔记：p551-p560

《Django 5 By Example》学习第 20 天，p551-p560 总结，总计 10 页。

一、技术总结

1.custom model field

(1)示例

courses/fields.py

from django.core.exceptions import ObjectDoesNotExist
from django.db import models


class OrderField(models.PositiveIntegerField):
    def __init__(self, for_fields=None, *args, **kwargs):
        self.for_fields = for_fields
        super().__init__(*args, **kwargs)

    def pre_save(self, model_instance, add):
        if getattr(model_instance, self.attname) is None:
            # no current value
            try:
                qs = self.model.objects.all()
                if self.for_fields:
                    query = {
                        field: getattr(model_instance, field)
                        for field in self.for_fields
                    }
                    qs = qs.filter(**query)
                # get the order of the last item
                last_item = qs.latest(self.attname)
                value = getattr(last_item, self.attname) + 1
            except ObjectDoesNotExist:
                value = 0
            setattr(model_instance, self.attname, value)
            return value
        else:
            return super().pre_save(model_instance, add)

courses/models.py：

from django.db import models
from django.contrib.auth.models import User
from django.contrib.contenttypes.fields import GenericForeignKey
from django.contrib.contenttypes.models import ContentType

from .fields import OrderField


# Create your models here.

class Subject(models.Model):
    title = models.CharField(max_length=200)
    slug = models.SlugField(max_length=200, unique=True)

    class Meta:
        ordering = ['title']

    def __str__(self):
        return self.title


class Course(models.Model):
    owner = models.ForeignKey(User, related_name='courses_created', on_delete=models.CASCADE)
    subject = models.ForeignKey(Subject, on_delete=models.CASCADE, related_name='courses')
    title = models.CharField(max_length=200)
    slug = models.SlugField(max_length=200, unique=True)
    overview = models.TextField()
    created = models.DateTimeField(auto_now_add=True)

    class Meta:
        ordering = ['created']

    def __str__(self):
        return self.title


class Module(models.Model):
    course = models.ForeignKey(
        Course, related_name='modules', on_delete=models.CASCADE
    )
    title = models.CharField(max_length=200)
    description = models.TextField(blank=True)
    order = OrderField(blank=True, for_fields=['course'])

    class Meta:
        ordering = ['order']

    def __str__(self):
        return f'{self.order}. {self.title}'


class Content(models.Model):
    module = models.ForeignKey(Module, related_name="contents", on_delete=models.CASCADE)
    content_type = models.ForeignKey(
        ContentType,
        on_delete=models.CASCADE,
        limit_choices_to={
            'model__in': ('text', 'video', 'image', 'file')
        }
    )
    object_id = models.PositiveIntegerField()
    item = GenericForeignKey('content_type', 'object_id')
    order = OrderField(blank=True, for_fields=['module'])

    class Meta:
        ordering = ['order']


class ItemBase(models.Model):
    owner = models.ForeignKey(User, related_name='%(class)s_related', on_delete=models.CASCADE)
    title = models.CharField(max_length=250)
    created = models.DateTimeField(auto_now_add=True)
    updated = models.DateTimeField(auto_now=True)

    class Meta:
        # 如果不设置 abstract = True 会有什么影响？
        abstract = True

    def __str__(self):
        return self.title


class Text(ItemBase):
    content = models.TextField()


class File(ItemBase):
    file = models.FileField(upload_to='files')


class Image(ItemBase):
    file = models.ImageField(upload_to='images')


class Video(ItemBase):
    url = models.URLField()

二、英语总结(生词：1)

1.in respect of

也写作 with respect to，意思是“in connection with something(与xxx有关的)”。

class Content(models.Model):

    # ...

    order = OrderField(blank=True, for_fields=['module'])

p557, This time, you specify that the order is calculated with respect to the module field(这一次，指定根据 module 字段计算顺序)。

三、其它

今天也没有什么想说的。

四、参考资料

1. 编程

(1) Antonio Melé，《Django 5 By Example》：https://book.douban.com/subject/37007362/

2. 英语

(1) Etymology Dictionary：https://www.etymonline.com

(2) Cambridge Dictionary：https://dictionary.cambridge.org

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