Any type of package needs to install for using many to many relationships in Django

If you are doing work with Django then learn this post it is very usefully

Any type of package needs to install for using many to many relationships in Django

Here...Any type of package needs to install for using many to many relationships in Django

There are some excellent idea to help you make the new concepts, you are learning

as a beginner to Any type of package needs to

install for using many to many relationships in Django explain step by step here...

=>Django application has a GM2MField that combines the features

of the standard Django “ManyToManyField” and “GenericForeignKey”.

=>Installation:

>> pip install django-gm2m

=>Django’s contenttype framework must be

(django.contrib.contenttypes) mentionned

in the INSTALLED_APPS.

INSTALLED_APPS = [

...

'django.contrib.contenttypes',

...

'gm2m',

]

=>Uses Of django-gm2m:

1.reverse relations

2.prefetching

3. Allows you to customize the deletion behaviour

In this example, an article can be published in multiple Publication

objects and a Publication has multiple Article objects:

from Django.db import models

class Publication(models.Model):

title = models.CharField(max_length=30)

class Meta:

ordering = ['title']

def __str__(self):

return self.title

class Article(models.Model):

headline = models.CharField(max_length=100)

publications = models.ManyToManyField(Publication)

class Meta:

ordering = ['headline']

def __str__(self):

return self.headline

What follows are examples of operations that can be performed using the Python API facilities.

Create a few Publications:

>>> p1 = Publication(title='The Python Journal')

>>> p1.save()

>>> p2 = Publication(title='Science News')

>>> p2.save()

>>> p3 = Publication(title='Science Weekly')

>>> p3.save()

=>Create an Article:

>>> a1 = Article(headline='Django lets you build Web apps easily')

You can’t associate it with a Publication until it’s been saved:

>>> a1.publications.add(p1)

Traceback (most recent call last):

ValueError: "<Article: Django lets you build Web apps easily>" needs to have a value for field "id" before this many-to-many relationship can be used.

Save it!

>>> a1.save()

Associate the Article with a Publication:

>>> a1.publications.add(p1)

Create another Article, and set it to appear in the Publications:

>>> a2 = Article(headline='NASA uses Python')

>>> a2.save()

>>> a2.publications.add(p1, p2)

>>> a2.publications.add(p3)

Adding a second time is OK, it will not duplicate the relation:

>>> a2.publications.add(p3)

Adding an object of the wrong type raises TypeError:

>>> a2.publications.add(a1)

Traceback (most recent call last):

TypeError: 'Publication' instance expected

Create and add a Publication to an Article in one step using create():

>>> new_publication = a2.publications.create(title='Highlights for Children')

Article objects have access to their related Publication objects:

>>> a1.publications.all()

>>> a2.publications.all()

Publication objects have access to their related Article objects:

>>> p2.article_set.all()

>>> p1.article_set.all()

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

Many-to-many relationships can be queried using lookups across relationships:

>>> Article.objects.filter(publications__id=1)

>>> Article.objects.filter(publications__pk=1)

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

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

>>> Article.objects.filter(publications__title__startswith="Science")

>>> Article.objects.filter(publications__title__startswith="Science").distinct()

The count() function respects distinct() as well:

>>> Article.objects.filter(publications__title__startswith="Science").count()

>>> Article.objects.filter(publications__title__startswith="Science").distinct().count()

>>> Article.objects.filter(publications__in=[1,2]).distinct()

>>> Article.objects.filter(publications__in=[p1,p2]).distinct()

Reverse m2m queries are supported (i.e., starting at

the table that doesn’t have a ManyToManyField):

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

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

>>> Publication.objects.filter(article__headline__startswith="NASA")

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

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

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

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

>>> Publication.objects.filter(article__in=[1,2]).distinct()

>>> Publication.objects.filter(article__in=[a1,a2]).distinct()

Excluding a related item works as you would expect, too (although the SQL involved is a little complex):

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

If we delete a Publication, its Articles won’t be able to access it:

>>> p1.delete()

>>> Publication.objects.all()

>>> a1 = Article.objects.get(pk=1)

>>> a1.publications.all()

If we delete an Article, its Publications won’t be able to access it:

>>> a2.delete()

>>> Article.objects.all()

>>> p2.article_set.all()

Adding via the ‘other’ end of an m2m:

>>> a4 = Article(headline='NASA finds intelligent life on Earth')

>>> a4.save()

>>> p2.article_set.add(a4)

>>> p2.article_set.all()

>>> a4.publications.all()

Adding via the other end using keywords:

>>> new_article = p2.article_set.create(headline='Oxygen-free diet works wonders')

>>> p2.article_set.all()

>>> a5 = p2.article_set.all()[1]

>>> a5.publications.all()

Removing Publication from an Article:

>>> a4.publications.remove(p2)

>>> p2.article_set.all()

>>> a4.publications.all()

And from the other end:

>>> p2.article_set.remove(a5)

>>> p2.article_set.all()

>>> a5.publications.all()

Relation sets can be set:

>>> a4.publications.all()

>>> a4.publications.set([p3])

>>> a4.publications.all()

Relation sets can be cleared:

>>> p2.article_set.clear()

>>> p2.article_set.all()

And you can clear from the other end:

>>> p2.article_set.add(a4, a5)

>>> p2.article_set.all()

>>> a4.publications.all()

>>> a4.publications.clear()

>>> a4.publications.all()

>>> p2.article_set.all()

Recreate the Article and Publication 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)

Bulk delete some Publications - references to deleted publications should go:

>>> Publication.objects.filter(title__startswith='Science').delete()

>>> Publication.objects.all()

>>> Article.objects.all()

>>> a2.publications.all()

Bulk delete some articles - references to deleted objects should go:

>>> q = Article.objects.filter(headline__startswith='Django')

>>> print(q)

>>> q.delete()

After the delete(), the QuerySet cache needs to be cleared, and the referenced objects should be gone:

>>> print(q)

>>> p1.article_set.all()

Conclusion-In this tutorial you will have learned how Any type of package needs to install for

using many to many relationships in Django

So hope you liked these tutorials. If you have any questions or suggestions related to Django please comment below and let us know.

Finally, if you find this post informative, then share it with

your friends on Facebook, Twitter, Instagram.

Thank you...

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