[PYTHON] [Django] About one-to-many relationships (related_name, _set.all ())

Introduction

There is a parameter called related_name in the argument of models.ForeignKey () used when associating models with each other in Django, but I did not know when it will be used, so I checked it.

Premise

This time, we will create a Category model and a Post model as follows.

apps/models.py

class Category(models.Model):
    name = models.CharField(max_length=100)
    slug = models.SlugField(unique=True)

    def __str__(self):
        return self.name

class Post(models.Model):
    category = models.ForeignKey(to=Category, on_delete=models.CASCADE)
    author = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
    title = models.CharField(max_length=100)
    content = models.TextField()

    def __str__(self):
        return self.title

Suppose that the above code generates 3 categories and 6 posts, and has the relationship shown in the following figure.

The following two cases are divided and each method is considered.

• When you want to get the category associated with the quadratic function (post.id = 1) << Refer to multiple → 1 >>
• When you want to get all the posts associated with society (category.id = 3) << 1 → Many references >>

How to refer to many → 1

Can be obtained by object.field name

>>> post1 = Post.object.get(id=1)
>>> post1
<Post:quadratic function>
>>> post1.category
<Category:Math>

1 → How to refer to many [_set.all ()]

Can be obtained with object.model name (lowercase) _set.all ()

>>> category3 = Category.object.get(id=3)
>>> category3
<Category:society>
>>> category3.post_set.all()
<QuerySet [<Post:Ryoma Sakamoto>, <Post:Kamakura Shogunate>]>

• In addition to .all (), you can also use .filter () and .count () to narrow down and count the number of objects.

What is related_name

• It is one of the arguments of models.ForeignKey and does not have to be separate.
• If related_name is specified, it can be used by replacing it with model name (lowercase) _set.
• If related_name = ‘posts’, in the previous example, category3.post_set.all() →→ category3.posts.all()
  It becomes more intuitive than . </ u>

class Category(models.Model):
    name = models.CharField(max_length=100)
    slug = models.SlugField(unique=True)

class Post(models.Model):
    category = models.ForeignKey(to=Category, on_delete=models.CASCADE, related_name='posts') #add to
    author = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
    title = models.CharField(max_length=100)
    content = models.TextField()

>>> category3 = Category.object.get(id=3)
>>> category3
<Category:society>
>>> category3.posts.all()　#Change
<QuerySet [<Post:Ryoma Sakamoto>, <Post:Kamakura Shogunate>]>　#← The result is the same