Django many-to-many relationships, django many-to-Relationship

Django many-to-many relationships, django many-to-Relationship

Django 1, 1.7

Use ManyToManyFiled to define multiple-to-multiple relationships.

In this example, the Article can be published on multiple publications, and the Publication can have multiple Article objects.

from django.db import modelsclass Publication(models.Model):    title = models.CharField(max_length=30)        def __str__(self):        return self.title        class Meta:        ordering = ('title',)class Article(models.Model):    headline = models.CharField(max_length=100)    publications = models.ManyToManyField(Publication)    def __str__(self):        return self.headline    class Meta:        ordering = ('headline',)

The following is an example of using Python APIs. Note: If you use an intermediate model for many-to-many relationships, some relational manager methods cannot be used, so some of these examples cannot work under this model.

Create several publications:

>>> P1 = Publication (title = 'the Python Journal ')

>>> P1.save ()

>>> P2 = Publication (title = 'science News ')

>>> P2.save ()

>>> P3 = Publication (title = 'science wekkly ')

Create an article:

>>> A1 = Article (headline = 'django lets you build web apps easily ')

Before saving it, it cannot be associated with a Publication.

>>> A1.publications. add (p1)

...

ValueError: 'Article' instance needs to have a primary key value before a failed-to-assign relationship can be used.

Save

>>> A1.save ()

Associate Article with Publication:

>>> A1.publications. add (p1)

Create another Article and set it to appear in two Publications:

>>> A2 = Article (headline = 'nasa uses python ')

>>> A2.save ()

>>> A2.publications. add (p1, p2)

>>> A2.publications. add (p3)

The 2nd add operation is correct:

>>> A2.publications. add (p3)

Adding an error type object produces a TypeError:

>>> A2.publictaions. add (a1)

Traceback (most recent call last ):

...

TypeError: 'publication' instance already ted

Use create () to create a Publications and add it to an article:

>>> New_publication = a2.publications. create (title = 'highlights for Children ')

Article objects can access their associated Publication objects:

>>> A1.publications. all ()

[<Publication: HIghlights for Children>,...]

>>> A2.publications. all ()

[<Publication: Highlights for Children>, <Publication: Science News>,...]

Publication objects can access their associated Article objects:

>>> P2.article _ set. all ()

[<Article: NASA uses Python>]

>>> P1.article _ set. all ()

[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]

>>> Publication. objects. get (id = 4). article_set.all ()

[<Article: NASA uses Python>]

You can use cross-link query to obtain multi-to-Multi-link queries:

>>> Article. objects. filter (publications _ id = 1)

[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]

>>> Article. objects. filter (publications _ pk = 1)

[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]

>>> Article. objects. filter (publications = 1 ):

[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]

>>> Article. objects. filter (publications = p1)

[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]

>>> Article. objects. filter (publications _ title _ startwith = 'science ')

[<Article: NASA uses Python>, <Article: NASA uses Python>]

>>> Article. objects. filter (publications _ title _ startwith = 'science '). distinct ()

[<Article: NASA uses Python>]

The count () function also uses distinct ():

>>> Article. objects. filter (publications _ title _ startwith = 'science '). count ()

2

>>> Article. objects. filter (publications _ title _ startwith = 'science '). distinct (). count ()

1

>>> Article. objects. filter (publications _ in = [1, 2]). distinct ()

[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]

>>> Article. objects. filter (publications _ in = [p1, p2]). distinct ()

[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]

Reverse m2m query is also supported (for example, starting from a table without the ManyToManyField field ):

>>> Publication. objects. filter (id = 1)

[<Publication: The Python Journal>]

>>> Publication. objects. filter (pk = 1)

[<Publication: The Python Journal>]

>>> Publication. objects. filter (article _ headline _ startwith = 'nasa ')

[<Publication: Highlights for Children>, <Publication: Science News>,...]

>>> Publication. objects. filter (article _ id = 1)

[<Publication: The Python Journal>]

>>> Publication. objects. filter (article _ pk = 1)

[<Publication: The Python Journal>]

>>> Publication. objects. filter (article = 1)

[<Publication: The Python Journal>]

>>> Publication. objects. filter (article = a1)

[<Publication: The Python Journal>]

Publication. objects. filter (article _ in = [1, 2]). distinct ()

[<Publication: Highlights for Children>, <Publication: Science News>,...]

>>> Publication. objects. filter (article _ in = [a1, a2]). distinct ()

[<Publication: Highlights for Children>, <Publication: Science News>,...]

Excluding an associated project will work as expected (although the SQL used is a little complicated ):

>>> Article. objects. exclude (publications = p2)

[<Article: Django lets you build Web apps easily>]

If we delete Publication, its Article will not be able to access it:

>>> P1.delete ()

>>> Publication. objects. all ()

[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly> ']

>>> A1 = Article. objects. get (pk = 1)

>>> A1.publications. all ()

[]

If we delete an Article, its Publications will not be able to access it:

>>> A2.delete ()

>>> Article. objects. all ()

[<Artilce: Django lets you bulid Web apps easily>]

>>> P2.article _ set. all ()

[]

Add through the other end of m2m:

>>> A4 = Article (headline = 'nasa finds intelligent lif on global ')

>>> A4.save ()

>>> P2.article _ set. add (a4)

>>> P2.article _ set. all ()

[<Article: NASA finds intelligent lif on Earth>]

>>> A4.publications. all ()

[<Publication: Science News>]

Add using keywords on the other end:

>>> New_article = p2.article _ set. create (headline = 'Oxygen-free diet works wonders ')

>>> P2.article _ set. all ()

[<Article: NASA finds intelligent lif on Earth>, <Article: Oxygen-free diet works wonders>]

>>> A5 = p2.article _ set. all () [1]

>>> A5.publications. all ()

[<Publication: Science News>]

Remove Publication from 1 Article:

>>> A4.publications. remove (p2)

>>> P2.article _ set. all ()

[<Article: Oxygen-free diet works wonders>]

>>> A4.publications. all ()

[]

From the other end:

>>> P2.article _ set. remove (a5)

>>> P2.article _ set. all ()

[]

>>> A5.publications. all ()

[]

You can assign values to a relational set. The assign value operation clears any existing set members:

>>> A4.publications. all ()

[<Publication: Science News>]

>>> A4.publications = [p3]

>>> A4.publications. all ()

[<Publication: Science Weekly>]

Link set can be cleared:

>>> P2.article _ set. clear ()

>>> P2.article _ set. all ()

[]

You can also clear from the other end:

>>> P2.article _ set. add (a4, a5)

>>> P2.article _ set. all ()

[<Article: NASA finds intelligent lif on Earth>, <Article: Oxygen-free diet works wonders>]

>>> A4.publications. clear ()

>>> A4.publications. all ()

[]

>>> P2.article _ set. all ()

[<Article: Oxygen-free diet works wonders>]

Re-create the articles and Publications we have deleted:

>>> P1 = Publication (title = 'the Python Journal ')

>>> P1.save ()

>>> A2 = Article (headline = 'nasa uses python ')

>>> A2.save ()

>>> A2.publications. add (p1, p2, p3)

Batch delete some Publications-reference deleted publications are also deleted:

>>> Publication. objects. filter (title _ startwith = 'science '). delete ()

>>> Publication. objects. all ()

[<Publication: Highlights for Children>, <Publication: The Python Journal>]

>>> Article. objects. all ()

[<Article: Django lets you build Web apps easily>,...]

>>> A2.publications. all ()

[<Publication: The Python Journal>]

Batch delete some articles-objects referenced and deleted are also deleted:

>>> Q = Article. objects. filter (headline _ startwith = 'django ')

>>> Print (q)

[<Article: Django lets you build Web apps easily>]

>>> Q. delete ()

After delete () is called, The QeruySet cache needs to be cleared, and the music object will also be deleted:

>>> Print (q)

[]

>>> P1.article _ set. all ()

[<Article: NASA uses Python>]

The alternative to calling clear () is to assign a null set:

>>> P1.article _ set = []

>>> P1.article _ set. all ()

>>> A2.publications = [p1, new_publication]

>>> A2.publications. all ()

[<Publication: Highlights for Children>, <Publication: The Python Journal>]

>>> A2.publications = []

>>> A2.publications. all ()

[]