我应该使用哪种类型的数据库关系？

Question

from django.db import models

class Reporter(models.Model):
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30)
    email = models.EmailField()

    def __str__(self):
        return "%s %s" % (self.first_name, self.last_name)

class Article(models.Model):
    headline = models.CharField(max_length=100)
    pub_date = models.DateField()
    reporter = models.ForeignKey(Reporter, on_delete=models.CASCADE)

    def __str__(self):
        return self.headline

    class Meta:
        ordering = ('headline',)

I found this one-to-many relationship example in the Django docs. How would I go about doing another class called Magazine. In the class magazine, it would contain many articles written by many reporters. Would I still use a many-to-one relationship, or would I need a one-to-one relationship?

Answer 1

That really depends on what you want to do... You have to be more specific, and tell us what your problem really is.

1. If you have n Articles in your Magazine, and each Article only occur in one Magazine, you have to use one-to-many.

2. If the Article can occur in multiple magazines, you have to use many-to-many relationship.

That's not actually a python question, by the way...

Answer 2

This is either a ForeignKey from Article to Magazine , or a ManyToManyField between Article and Magazine . This depends on whether an article can occur in multiple magazines (frequently journalists sell the same article to different magazines, those are then, except for some formatting, word-by-word the same).

In case every article occurs in one magazine, then we thus implement it like:

class Article(models.Model):
    headline = models.CharField(max_length=100)
    pub_date = models.DateField()
    reporter = models.ForeignKey(Reporter, on_delete=models.CASCADE)
    

    def __str__(self):
        return self.headline

    class Meta:
        ordering = ('headline',)

In case an article can remain unpublished , then you also probably have to set null=True , in the ForeignKey constructor.

In case of a ManyToManyField , django will implicitly create a table like article_magazine that stores mappings from Article s to Magazine s (but if you query, you will only obtain the Magazine s).

The relation is very probably not a OneToOneField . In fact a OneToOneField is a ForeignKey with unique=True . This thus means that no two Article s are published in the same Magazine . Therefore it means that every Magazine has either no or exactly one Article related to it.

Answer 3

According to your need i think you should refer to Many-to-many relationship .

1. In you think, an Article can be published using multiple reporter objects, and a Reporter has multiple Article objects then refer to https://docs.djangoproject.com/en/2.0/topics/db/examples/many_to_many/

2. If you Logic supports one reporter per article and multiple article per reporter then visit: https://docs.djangoproject.com/en/2.0/topics/db/examples/many_to_one/ .