Untuk menentukan hubungan many-to-one, gunakan ForeignKey
:
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']
Apa yang mengikuti adalah contoh-contoh dari tindakan yang dapat dilakukan menggunakan fasilitas API Python.
Buat sedikir Reporter:
>>> r = Reporter(first_name='John', last_name='Smith', email='john@example.com')
>>> r.save()
>>> r2 = Reporter(first_name='Paul', last_name='Jones', email='paul@example.com')
>>> r2.save()
Buat sebuah Artikel:
>>> from datetime import date
>>> a = Article(id=None, headline="This is a test", pub_date=date(2005, 7, 27), reporter=r)
>>> a.save()
>>> a.reporter.id
1
>>> a.reporter
<Reporter: John Smith>
Catat bahwa anda harus menyimpan sebuah obyek sebelum itu dapat diberikan ke hubungan foreign key. Sebagai contoh, membuat Article
dengan Reporter
tidak disimpan memunculkan ValueError
:
>>> r3 = Reporter(first_name='John', last_name='Smith', email='john@example.com')
>>> Article.objects.create(headline="This is a test", pub_date=date(2005, 7, 27), reporter=r3)
Traceback (most recent call last):
...
ValueError: save() prohibited to prevent data loss due to unsaved related object 'reporter'.
Obyek artikel mempunyai akses ke obyek Reporter terkait:
>>> r = a.reporter
Buat sebuah Article melalui obyek Reporter:
>>> new_article = r.article_set.create(headline="John's second story", pub_date=date(2005, 7, 29))
>>> new_article
<Article: John's second story>
>>> new_article.reporter
<Reporter: John Smith>
>>> new_article.reporter.id
1
Buat sebuah artikel baru:
>>> new_article2 = Article.objects.create(headline="Paul's story", pub_date=date(2006, 1, 17), reporter=r)
>>> new_article2.reporter
<Reporter: John Smith>
>>> new_article2.reporter.id
1
>>> r.article_set.all()
<QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>
Tambah artikel sama ke kumpulan artikel berbeda - periksa bahwa itu berpindah:
>>> r2.article_set.add(new_article2)
>>> new_article2.reporter.id
2
>>> new_article2.reporter
<Reporter: Paul Jones>
Menambahkan sebuah obyek dari jenis salah memunculkan TypeError:
>>> r.article_set.add(r2)
Traceback (most recent call last):
...
TypeError: 'Article' instance expected, got <Reporter: Paul Jones>
>>> r.article_set.all()
<QuerySet [<Article: John's second story>, <Article: This is a test>]>
>>> r2.article_set.all()
<QuerySet [<Article: Paul's story>]>
>>> r.article_set.count()
2
>>> r2.article_set.count()
1
Catat bahwa di contoh terakhir artikel telah dipindahkan dari John ke Paul.
Pengelola terkait mendukung pencarian bidang-bidang juga. API otomatis mengikuti hubungan sejauh anda butuhkan. Gunakan garis bawah ganda untuk memisahkan hubungan. Ini bekerja sebanyak tingkatan yang anda inginkan. Tidak ada batasan. Sebagai contoh:
>>> r.article_set.filter(headline__startswith='This')
<QuerySet [<Article: This is a test>]>
# Find all Articles for any Reporter whose first name is "John".
>>> Article.objects.filter(reporter__first_name='John')
<QuerySet [<Article: John's second story>, <Article: This is a test>]>
Benar-benar cocok tersirat disini:
>>> Article.objects.filter(reporter__first_name='John')
<QuerySet [<Article: John's second story>, <Article: This is a test>]>
Permintaan dua kali lebih bidang terkait. Ini menterjemahkan pada sebuah keadaan AND dalam klausa WHERE:
>>> Article.objects.filter(reporter__first_name='John', reporter__last_name='Smith')
<QuerySet [<Article: John's second story>, <Article: This is a test>]>
Untuk pencarian terkait anda dapat menyediakan sebuah nilai primary key atau lewatkan obyek terkait dengan tegas:
>>> Article.objects.filter(reporter__pk=1)
<QuerySet [<Article: John's second story>, <Article: This is a test>]>
>>> Article.objects.filter(reporter=1)
<QuerySet [<Article: John's second story>, <Article: This is a test>]>
>>> Article.objects.filter(reporter=r)
<QuerySet [<Article: John's second story>, <Article: This is a test>]>
>>> Article.objects.filter(reporter__in=[1,2]).distinct()
<QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>
>>> Article.objects.filter(reporter__in=[r,r2]).distinct()
<QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>
Anda dapat juga menggunakan queryset daripada daftar harfiah dari instance:
>>> Article.objects.filter(reporter__in=Reporter.objects.filter(first_name='John')).distinct()
<QuerySet [<Article: John's second story>, <Article: This is a test>]>
Meminta di arah berlawanan:
>>> Reporter.objects.filter(article__pk=1)
<QuerySet [<Reporter: John Smith>]>
>>> Reporter.objects.filter(article=1)
<QuerySet [<Reporter: John Smith>]>
>>> Reporter.objects.filter(article=a)
<QuerySet [<Reporter: John Smith>]>
>>> Reporter.objects.filter(article__headline__startswith='This')
<QuerySet [<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]>
>>> Reporter.objects.filter(article__headline__startswith='This').distinct()
<QuerySet [<Reporter: John Smith>]>
Menghitung dalam arah berlawanan bekerja dalam hubungan dengan distinct():
>>> Reporter.objects.filter(article__headline__startswith='This').count()
3
>>> Reporter.objects.filter(article__headline__startswith='This').distinct().count()
1
Permintaan dapat berputar seperti lingkaran:
>>> Reporter.objects.filter(article__reporter__first_name__startswith='John')
<QuerySet [<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]>
>>> Reporter.objects.filter(article__reporter__first_name__startswith='John').distinct()
<QuerySet [<Reporter: John Smith>]>
>>> Reporter.objects.filter(article__reporter=r).distinct()
<QuerySet [<Reporter: John Smith>]>
Jika anda menghapus sebuah reporter, artikel dia akan dihapus (menganggap bahwa ForeignKey telah ditentukan dengan django.db.models.ForeignKey.on_delete
setel menjadi CASCADE
, yang merupakan awalan):
>>> Article.objects.all()
<QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>
>>> Reporter.objects.order_by('first_name')
<QuerySet [<Reporter: John Smith>, <Reporter: Paul Jones>]>
>>> r2.delete()
>>> Article.objects.all()
<QuerySet [<Article: John's second story>, <Article: This is a test>]>
>>> Reporter.objects.order_by('first_name')
<QuerySet [<Reporter: John Smith>]>
Anda dapat menghapus menggunakan JOIN di permintaan:
>>> Reporter.objects.filter(article__headline__startswith='This').delete()
>>> Reporter.objects.all()
<QuerySet []>
>>> Article.objects.all()
<QuerySet []>
Des 02, 2019