Relationships in Django ORM

1. Relationships in Django ORM
@starwilly

2. Outline
• Single model ( Django girls tutorial )
• Relationship
• 1. many-to-one ( )
• 2. many-to-many (Tag)
• What does it looks like in database
• How to define relationship
• Traverse / query / create / delete relation

3. A Simple Django Model

4. CRUD (Create, Read, Update, Delete)
Post.objects.create(title='My First Trip',
content=‘ ?',
location=' ')
Post.objects.all()
[<Post: My First Trip>, <Post: My Second Trip>, <Post:
Django >]
Post.objects.filter(pk__gt=1)
[<Post: My Second Trip>, <Post: Django >]

5. CRUD
posts = Post.objects.filter(pk__lt=3)
[<Post: My First Trip>, <Post: My Second Trip>]
posts.update(location=‘ ')
2
posts.delete()

6. Model : Table
id title content photo location created_at
1 …
2 …

7. Relationship
• many-to-one
• many-to-many

8. Many-to-one Relationship

9. •
•
• 1.
• 2.
Alan:
Bob:
Cindy:

10. Model

11. Many-to-one Relationship
•
PostComment
•
1m

12. ForeignKey
• (many-to-one)
ForeignKey(othermodel, **options)

13. Use Foreign Key to Define m:1
Comment Post

14. id author content
1 Alan …
2 Bob …
3 Cindy …
4 Denny
5
id title content
1 …
2 …

15. id post_id author content
1 1 Alan …
2 1 Bob …
3 1 Cindy …
4 2 Denny
5 2 Alan
id title content
1 …
2 …

16. Query m:1 Relationship

17. id = 1
Alan:
Bob:
Cindy:

18. Traverse many-to-one relationship
id (pk) = 1
post1 = Post.objects.get(pk=1)
comments = Comment.objects.filter(post_id=1)
comments = Comment.objects.filter(post=post1)
[<Comment: Alan: >, <Comment: Bob: >, <Comment:
Cindy: >]

19. Traverse many-to-one relationship
id (pk) = 1
post1 = Post.objects.get(pk=1)
comments = post1.comment_set.all()
<Model>_set
[<Comment: Alan: >, <Comment: Bob: >, <Comment:
Cindy: >]

20. Associate Post and Comment
post = Post.objects.get(pk=1)
comment = Comment(author=‘Billy’,
content=‘hi’)
post.comment_set.add(comment)

21. Remove Comment from Post
post1 = Post.objects.get(pk=1)
post1.comment_set.all()
[<Comment: Alan: >, <Comment: Bob: >, <Comment:
Cindy: >]
c = Comment.objects.get(pk=1)
<Comment: Alan: >
c.delete()
post.comment_set.all()
[<Comment: Bob: >, <Comment: Cindy: >]

22. ForeignKey.related_name
comment_list = post.comment_set.all()
comment_list = post.comments.all()

23. Foreign Key
• (many-to-one relationship)
• <model>_set
• related_name
• add()

24. Many-to-many Relationships

25. Tag
myblog.com/post/1 myblog.com/tag/food

26. What is Many-to-many Relationship
Post Tag
• tag
• tag
n m

27. Tag Model

28. SlugField
• CharField
• (hyphen)
•
• myblog.com/tags/ (X)
• myblog.com/tags/food (O)
• myblog.com/posts/[ ] (X)
• myblog.com/posts/a-wonderful-trip-in-japan (O)

29. Tag Table
id name slug
1 food
2 japan
3 taiwan-north
4 taipei
… …
taipei

30. id title content
1 …
2 …
Post
Tag
, ,
id name slug
1 food
2 japan
3 taiwan-north
4 taipei
… …

31. id title content tags
1 … 1, 3, 4
2 … 2
Post
Tag
, ,
id name slug
1 food
2 japan
3 taiwan-north
4 taipei
… …

32. id title content
1 …
2 …
Post
Tag
, ,
id name slug
1 food
2 japan
3 taiwan-north
4 taipei
… …
id post_id tag_id
1 1 1
2 2 2
3 1 3
4 1 4

33. ManyToManyField
ManyToManyField(othermodel, **options)
tags = models.ManyToManyField('Tag', blank=True)

34. ManyToManyField
•
• Django m2m
ManyToManyField(othermodel, **options)

35. id title content
1 …
2 …
id name slug
1 food
2 japan
… … …
id post_id tag_id
1 1 1
2 2 2
3 1 3
4 1 4

36. Query M2M Relationship
from trips.models import Post, Tag
post1 = Post.objects.get(pk=1)
<Post: >
post1.tags.all()
[<Tag: >, <Tag: >]

37. Query M2M Relationship
from trips.models import Post, Tag
tag_food = Tag.objects.get(pk=1)
<Tag: >
tag_food.post_set.all()
[<Post: >]

38. ManyToManyField.related_name
tag_food = Tag.objects.get(pk=1)
tag_food.post_set.all()
tag_food.posts.all()

39. Create M2M Relationship
from trips.models import Post, Tag
post1 = Post.objects.get(pk=1)
tag_japan = Tag.objects.create(name=‘ ’,
slug=‘japan’)
tag_food = Tag.objects.get(slug=‘food’)
post1.tags.add(tag_japan)
tag_food.post_set.add(post1)

40. Remove M2M Relationship
from trips.models import Post, Tag
tag_japan = Tag.objects.get(slug=‘japan’)
post1 = Post.objects.get(pk=1)
post1.tags.remove(tag_japan)
tag_japan.post_set.remove(post1)

41. Filter M2M Relationship
from trips.models import Post, Tag
tag_japan = Tag.objects.get(slug=‘japan’)
tag_food = Tag.objects.get(slug=‘food’)
tag_japan.post_set.all()
# [<Post: >]
Post.objects.filter(tags=tag_japan)
# [<Post: >]
Post.objects.filter(tags__in=[tag_japan, tag_food])
#[<Post: >, <Post: >]

42. ManyToManyField
•
• Table (Mapping)
• releated_name
• add() , remove()
• https://docs.djangoproject.com/en/1.9/ref/models/relations/

43. Get your hand dirty
• Tag
• Tag
•
• Tag
• Tag Tag

44. Tips:
djangogirls/mysite/templates/post.html

45. Tips: Tag
djangogirls/mysite/templates/post.html

46. Tips: Tag
url(r'^tag/(?P<slug>[w-]+)/$', tag_detail, name='tag_detail'),
djangogirls/mysite/trips/views.py
djangogirls/mysite/templates/tag.html