ISOM 350

Business Application Development

Mohammad AlMarzouq

Traversing Relationship

Relationship in Django

• Represented in models using
  • ForeignKey
  • OneToOneField
  • ManyToMany Field

Updating our Blog App

• Each blog post will have multiple comments
• Comment author will provide comment, optional name, and optional email
• Comments will be displayed along with author name in order below the post
• Date/Time of comment will be displayed next to it
• Number of comments and date/time of latest comment will be shown next to each post in post list

Updated ERD

erDiagram
    POST ||--o{ COMMENT : Has

    POST {
        string title
        string slug
        string body
        datetime created_on
        datetime updated_on
        int status 
    }

    COMMENT{
        string comment
        string author
        string email
        datetime created_on
    }
    

Updated Models.py

class Comment(models.Model):
  comment = models.TextField()
  author = models.CharField(max_length=100, blank=True, null=True)
  email = models.EmailField(blank=True, null=True)
  created_on = models.DateTimeField(auto_now_add=True)
  post = models.ForeignKey('Post', on_delete=models.CASCADE) 

Admin and Migrations

• Create an inline Admin for the Post
• Don’t forget makemigrations and migrate, why?
• Create at least 2 posts
• Create 2 comments for 1st post, and 3 comments for 2nd post.

Traversing Relationship

• How do we display comments for each post?

From Comment to Post

• The Many to One direction
• Just reference the relationship field:

comment = Comment.objects.get(pk=id)
post = comment.post

• The post field will give you a Post object

From Post to Comment

• The One to Many direction
• Use reverse relationships
• For every relationship Django makes available a field in the related model to allow moving in the other direction

From Post to Comment

• For ForeignKey the reverse relationship gets the name model_set, for example comment_set in the current example
• It’s just a model manager like objects, you can use all() and filter() on it
• Everything you learned about fetching objects using all and filter applies here as well

post = Post.objects.get(pk=pid)
comments = post.comment_set.all()

• comments will include only the comments that belong to the post object in this example
• comments will be a list of objects

The Reverse Relation Model Manager

• Everything you learned about the objects model manager applies
  • You can use all, filter, and get
  • also update, create, and delete (yet to be covered)
  • Applies to ForeignKeyField, ManyToManyField, and OneToOneField, but slightly different
  • Read the documentation on model fields

Modifying The Reverse Relationship Name

• The attribute name by default is modelname_set
• Can be changed using the related_name option in Comment mode in models.py

  post = models.ForeignKey('Post', 
    on_delete=models.CASCADE, 
    related_name="comments") 

• Will replace post.comment_set with post.comments
• Useful for ManyToMany or multiple relationships of the same type

Remember

• When traversing relationships, you are just fetching related data.
• Everything else about the view is just the same

Updated Blog views.py

def detailed_post_view(request, slug):
  data = {}
  post = Post.objects.get(slug=slug)
  data["post"] = post
  data["comments"] = post.comment_set.all()
  return render(request, "detailed_post.html", context=data)

• What if you wanted to filter comments to exclude ones without a name?

detailed_post.html Template

 <h1>{{ post.title}}</h1>
  <ul>
    <li>Posted on: {{ post.created_on }} </li>
    <li>Last updated: {{ post.updated_on }} </li>
  </ul>
  <p>
    {{ post.body }}
  </p>
  <h2>comments:</h2>
  <ul>
  {% for c in comments %}
    <li>{{ c.author }}: {{ c.comment }}</li>
  {% endfor %}
  </ul>

The Result

Do not forget to wire the urls