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Django ORM untuk Pemula | Kami mengoptimalkan permintaan



Django ORM (Object Relational Mapping) adalah salah satu fitur paling kuat dari Django. Ini memungkinkan kita untuk berinteraksi dengan database menggunakan kode Python, bukan SQL.

Untuk menunjukkan, saya akan menjelaskan model berikut:

from django.db import models

class Blog(models.Model):
    name = models.CharField(max_length=250)
    url = models.URLField()

    def __str__(self):
        return self.name

class Author(models.Model):
    name = models.CharField(max_length=250)

    def __str__(self):
        return self.name

class Post(models.Model):
    title = models.CharField(max_length=250)
    content = models.TextField()
    published = models.BooleanField(default=True)
    blog = models.ForeignKey(Blog, on_delete=models.CASCADE)
    authors = models.ManyToManyField(Author, related_name="posts")

Saya akan menggunakan django-extions untuk mendapatkan informasi yang berguna dengan:

python manage.py shell_plus --print-sql

Jadi, mari kita mulai:

>>> post = Post.objects.all()
>>> post
SELECT "blog_post"."id",
       "blog_post"."title",
       "blog_post"."content",
       "blog_post"."blog_id"
  FROM "blog_post"
 LIMIT 21
Execution time: 0.000172s [Database: default]
<QuerySet [<Post: Post object (1)>]>

1. Gunakan nilai ForeignKey secara langsung


>>> Post.objects.first().blog.id
SELECT "blog_post"."id",
       "blog_post"."title",
       "blog_post"."content",
       "blog_post"."blog_id"
  FROM "blog_post"
 ORDER BY "blog_post"."id" ASC
 LIMIT 1
Execution time: 0.000225s [Database: default]
SELECT "blog_blog"."id",
       "blog_blog"."name",
       "blog_blog"."url"
  FROM "blog_blog"
 WHERE "blog_blog"."id" = 1
 LIMIT 21
Execution time: 0.000144s [Database: default]
1

Dan jadi kami mendapatkan 1 permintaan dalam database:

>>> Post.objects.first().blog_id
SELECT "blog_post"."id",
       "blog_post"."title",
       "blog_post"."content",
       "blog_post"."blog_id"
  FROM "blog_post"
 ORDER BY "blog_post"."id" ASC
 LIMIT 1
Execution time: 0.000155s [Database: default]
1

2. Hubungan OneToMany


Jika kami menggunakan hubungan OneToMany, kami menggunakan bidang ForeignKey dan kueri terlihat seperti ini:

>>> post = Post.objects.get(id=1)
SELECT "blog_post"."id",
       "blog_post"."title",
       "blog_post"."content",
       "blog_post"."blog_id"
  FROM "blog_post"
 WHERE "blog_post"."id" = 1
 LIMIT 21
Execution time: 0.000161s [Database: default]

Dan jika kita ingin mengakses objek blog dari objek posting, kita dapat melakukan:

>>> post.blog
SELECT "blog_blog"."id",
       "blog_blog"."name",
       "blog_blog"."url"
  FROM "blog_blog"
 WHERE "blog_blog"."id" = 1
 LIMIT 21
Execution time: 0.000211s [Database: default]
<Blog: Django tutorials>

Namun, ini memicu permintaan baru untuk mendapatkan informasi dari blog. Jadi gunakan select_related untuk menghindari ini. Untuk menggunakannya, kami dapat memperbarui permintaan awal kami:

>>> post = Post.objects.select_related("blog").get(id=1)
SELECT "blog_post"."id",
       "blog_post"."title",
       "blog_post"."content",
       "blog_post"."blog_id",
       "blog_blog"."id",
       "blog_blog"."name",
       "blog_blog"."url"
  FROM "blog_post"
 INNER JOIN "blog_blog"
    ON ("blog_post"."blog_id" = "blog_blog"."id")
 WHERE "blog_post"."id" = 1
 LIMIT 21
Execution time: 0.000159s [Database: default]

Harap dicatat bahwa Django menggunakan GABUNG sekarang! Dan waktu eksekusi permintaan lebih pendek dari sebelumnya. Selain itu, sekarang post.blog akan di-cache!

>>> post.blog
<Blog: Django tutorials>

select_related juga berfungsi dengan QurySets:

>>> posts = Post.objects.select_related("blog").all()
>>> for post in posts:
...     post.blog
...
SELECT "blog_post"."id",
       "blog_post"."title",
       "blog_post"."content",
       "blog_post"."blog_id",
       "blog_blog"."id",
       "blog_blog"."name",
       "blog_blog"."url"
  FROM "blog_post"
 INNER JOIN "blog_blog"
    ON ("blog_post"."blog_id" = "blog_blog"."id")
Execution time: 0.000241s [Database: default]
<Blog: Django tutorials>

3. Banyak Hubungan


Untuk mendapatkan penulis pos, kami menggunakan sesuatu seperti ini:

>>> for post in Post.objects.all():
...     post.authors.all()
...
SELECT "blog_post"."id",
       "blog_post"."title",
       "blog_post"."content",
       "blog_post"."blog_id"
  FROM "blog_post"
Execution time: 0.000242s [Database: default]
SELECT "blog_author"."id",
       "blog_author"."name"
  FROM "blog_author"
 INNER JOIN "blog_post_authors"
    ON ("blog_author"."id" = "blog_post_authors"."author_id")
 WHERE "blog_post_authors"."post_id" = 1
 LIMIT 21
Execution time: 0.000125s [Database: default]
<QuerySet [<Author: Dmytro Parfeniuk>, <Author: Will Vincent>, <Author: Guido van Rossum>]>
SELECT "blog_author"."id",
       "blog_author"."name"
  FROM "blog_author"
 INNER JOIN "blog_post_authors"
    ON ("blog_author"."id" = "blog_post_authors"."author_id")
 WHERE "blog_post_authors"."post_id" = 2
 LIMIT 21
Execution time: 0.000109s [Database: default]
<QuerySet [<Author: Dmytro Parfeniuk>, <Author: Will Vincent>]>

Sepertinya kami mendapat permintaan untuk setiap objek posting. Karena itu, kita harus menggunakan prefetch_related . Ini mirip dengan select_related tetapi digunakan dengan ManyToMany Fields:

>>> for post in Post.objects.prefetch_related("authors").all():
...     post.authors.all()
...
SELECT "blog_post"."id",
       "blog_post"."title",
       "blog_post"."content",
       "blog_post"."blog_id"
  FROM "blog_post"
Execution time: 0.000300s [Database: default]
SELECT ("blog_post_authors"."post_id") AS "_prefetch_related_val_post_id",
       "blog_author"."id",
       "blog_author"."name"
  FROM "blog_author"
 INNER JOIN "blog_post_authors"
    ON ("blog_author"."id" = "blog_post_authors"."author_id")
 WHERE "blog_post_authors"."post_id" IN (1, 2)
Execution time: 0.000379s [Database: default]
<QuerySet [<Author: Dmytro Parfeniuk>, <Author: Will Vincent>, <Author: Guido van Rossum>]>
<QuerySet [<Author: Dmytro Parfeniuk>, <Author: Will Vincent>]>

Apa yang baru saja terjadi ??? Kami mengurangi jumlah kueri dari 2 menjadi 1 untuk mendapatkan 2 QuerySet-a!

4. Ambil sebelumnya objek


prefetch_related cukup untuk sebagian besar kasus, tetapi itu tidak selalu membantu untuk menghindari permintaan tambahan. Misalnya, jika kita menggunakan pemfilteran, Django tidak dapat menggunakan posting cache kita , karena mereka tidak difilter ketika mereka diminta dalam permintaan pertama. Dan kita mendapatkan:

>>> authors = Author.objects.prefetch_related("posts").all()
>>> for author in authors:
...     print(author.posts.filter(published=True))
...
SELECT "blog_author"."id",
       "blog_author"."name"
  FROM "blog_author"
Execution time: 0.000580s [Database: default]
SELECT ("blog_post_authors"."author_id") AS "_prefetch_related_val_author_id",
       "blog_post"."id",
       "blog_post"."title",
       "blog_post"."content",
       "blog_post"."published",
       "blog_post"."blog_id"
  FROM "blog_post"
 INNER JOIN "blog_post_authors"
    ON ("blog_post"."id" = "blog_post_authors"."post_id")
 WHERE "blog_post_authors"."author_id" IN (1, 2, 3)
Execution time: 0.000759s [Database: default]
SELECT "blog_post"."id",
       "blog_post"."title",
       "blog_post"."content",
       "blog_post"."published",
       "blog_post"."blog_id"
  FROM "blog_post"
 INNER JOIN "blog_post_authors"
    ON ("blog_post"."id" = "blog_post_authors"."post_id")
 WHERE ("blog_post_authors"."author_id" = 1 AND "blog_post"."published" = 1)
 LIMIT 21
Execution time: 0.000299s [Database: default]
<QuerySet [<Post: Post object (1)>, <Post: Post object (2)>]>
SELECT "blog_post"."id",
       "blog_post"."title",
       "blog_post"."content",
       "blog_post"."published",
       "blog_post"."blog_id"
  FROM "blog_post"
 INNER JOIN "blog_post_authors"
    ON ("blog_post"."id" = "blog_post_authors"."post_id")
 WHERE ("blog_post_authors"."author_id" = 2 AND "blog_post"."published" = 1)
 LIMIT 21
Execution time: 0.000336s [Database: default]
<QuerySet [<Post: Post object (1)>, <Post: Post object (2)>]>
SELECT "blog_post"."id",
       "blog_post"."title",
       "blog_post"."content",
       "blog_post"."published",
       "blog_post"."blog_id"
  FROM "blog_post"
 INNER JOIN "blog_post_authors"
    ON ("blog_post"."id" = "blog_post_authors"."post_id")
 WHERE ("blog_post_authors"."author_id" = 3 AND "blog_post"."published" = 1)
 LIMIT 21
Execution time: 0.000412s [Database: default]
<QuerySet [<Post: Post object (1)>]>

Artinya, kami menggunakan prefetch_related untuk mengurangi jumlah permintaan, tetapi kami sebenarnya meningkatkannya. Untuk menghindari ini, kami dapat menyesuaikan permintaan menggunakan objek Prefetch :

>>> authors = Author.objects.prefetch_related(
...     Prefetch(
...             "posts",
...             queryset=Post.objects.filter(published=True),
...             to_attr="published_posts",
...     )
... )
>>> for author in authors:
...     print(author.published_posts)
...
SELECT "blog_author"."id",
       "blog_author"."name"
  FROM "blog_author"
Execution time: 0.000183s [Database: default]
SELECT ("blog_post_authors"."author_id") AS "_prefetch_related_val_author_id",
       "blog_post"."id",
       "blog_post"."title",
       "blog_post"."content",
       "blog_post"."published",
       "blog_post"."blog_id"
  FROM "blog_post"
 INNER JOIN "blog_post_authors"
    ON ("blog_post"."id" = "blog_post_authors"."post_id")
 WHERE ("blog_post"."published" = 1 AND "blog_post_authors"."author_id" IN (1, 2, 3))
Execution time: 0.000404s [Database: default]
[<Post: Post object (1)>, <Post: Post object (2)>]
[<Post: Post object (1)>, <Post: Post object (2)>]
[<Post: Post object (1)>]

Kami menggunakan permintaan khusus untuk menerima posting melalui parameter permintaan dan menyimpan pesan yang difilter dalam atribut baru. Seperti yang bisa kita lihat, sekarang kita hanya punya 2 query ke database.

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