ORM🏹: 장고 ManyToManyField

./blog/models.py

from django.db import models


class Blog(models.Model):
    name = models.CharField(max_length=100)
    tagline = models.TextField()

    def __str__(self):
        return self.name


class Author(models.Model):
    name = models.CharField(max_length=200)
    email = models.EmailField()

    def __str__(self):
        return self.name


class Entry(models.Model):
    blog = models.ForeignKey(Blog, on_delete=models.CASCADE)
    headline = models.CharField(max_length=255)
    body_text = models.TextField()
    pub_date = models.DateField()
    mod_date = models.DateField()
    authors = models.ManyToManyField(Author)
    number_of_comments = models.IntegerField()
    number_of_pingbacks = models.IntegerField()
    rating = models.IntegerField()

    def __str__(self):
        return self.headline

위의 모델을 migrate하면 DB에는 어떤 테이블들이 생길까?

• blog_blog
• blog_author
• blog_entry

그리고 Entry에 ManyToManyField 설정을 생긴!

• blog_entry_authors

• blog_blog
  CREATE TABLE "blog_blog" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "name" varchar(100) NOT NULL, "tagline" text NOT NULL)
• blog_author
  CREATE TABLE "blog_author" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "name" varchar(200) NOT NULL, "email" varchar(254) NOT NULL)
• blog_entry
  CREATE TABLE "blog_entry" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT,
  "headline" varchar(255) NOT NULL,
  "body_text" text NOT NULL,
  "pub_date" date NOT NULL, "mod_date" date NOT NULL,
  "number_of_comments" integer NOT NULL, "number_of_pingbacks" integer NOT NULL, "rating" integer NOT NULL,
  "blog_id" integer NOT NULL REFERENCES "blog_blog" ("id") DEFERRABLE INITIALLY DEFERRED)
• blog_entry_authors
  CREATE TABLE "blog_entry_authors" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT,
  "entry_id" integer NOT NULL REFERENCES "blog_entry" ("id") DEFERRABLE INITIALLY DEFERRED,
  "author_id" integer NOT NULL REFERENCES "blog_author" ("id") DEFERRABLE INITIALLY DEFERRED)

ManyToManyField은 해당 필드로 연결된 2개의 테이블을 위한 중간 테이블은 만들어 준당!

• Entry테이블에서 authors에 ManyToManyField를 통해서 author를 연결해줌!
  
• 그랬더니 "blog_entry_authors"라는 중간 테이블 탄생! 짠!
  
• 생성되는 테이블을 좀 내맘대로 커스터마이징하고 싶다면 "through"를 씁니당!

ManyToManyField에 데이터 집어넣는 방법은 바로 add 메소드!

e = Entry.objects.get(pk=1)
b = Blog.objects.get(pk=1)
e.blog = b
e.save()

bob = Author.objects.create(name="Bob")
e.authors.add(bob)

add 메소드는 ManyToManyField로 지정된 속성에만 나타나는 메소드이당!

ManyToManyField.through_fields

기타

Reference

• https://medium.com/@dlarkqrl4966/%EB%8C%80%EA%B7%9C%EB%AA%A8-django-%EC%95%B1-%EA%B5%AC%EC%B6%95%EC%9D%84%EC%9C%84%ED%95%9C-%ED%8C%81-cdd2138afec1
• https://docs.djangoproject.com/en/3.1/topics/db/queries/
• https://docs.djangoproject.com/en/3.1/ref/models/fields/#manytomanyfield
• https://docs.djangoproject.com/en/3.1/ref/models/fields/#django.db.models.ManyToManyField.through_fields