Relationships

ForeignKey (Many-to-One)

class Comment(models.Model):
    post   = models.ForeignKey("Post", on_delete="CASCADE")
    body   = models.TextField()

Querying:

# Get all comments for a post
comments = await Comment.objects.filter(post_id=post.id).all()

# Get the post for a comment (access the FK id directly)
comment = await Comment.objects.get(id=1)
post    = await Post.objects.get(id=comment.post_id)

OneToOneField

class Profile(models.Model):
    user   = models.OneToOneField("User", on_delete="CASCADE")
    bio    = models.TextField()
    avatar = models.URLField()

ForeignKey options

models.ForeignKey(
    "Post",
    on_delete  = models.CASCADE,   # see table below
    null       = False,
    db_index   = True,             # adds an index on the FK column (default: True)
)

on_delete constants

Constant	SQL clause	Notes
`CASCADE`	`ON DELETE CASCADE`	Python callbacks fire (signals, pre_delete)
`DB_CASCADE`	`ON DELETE CASCADE`	Database handles deletion — no Python involved
`SET_NULL`	`ON DELETE SET NULL`	Python callbacks fire; requires `null=True`
`DB_SET_NULL`	`ON DELETE SET NULL`	Database sets NULL — no Python involved
`SET_DEFAULT`	`ON DELETE SET DEFAULT`	Python callbacks fire
`DB_SET_DEFAULT`	`ON DELETE SET DEFAULT`	Database sets default — no Python involved
`PROTECT`	`ON DELETE RESTRICT`	Raises an error if related rows exist
`RESTRICT`	`ON DELETE RESTRICT`	Same as PROTECT
`DO_NOTHING`	`ON DELETE NO ACTION`	No enforcement at all

DB_* variants are preferred in async applications. They skip loading related objects into Python memory, making bulk deletes orders of magnitude faster.

class Comment(models.Model):
    post = models.ForeignKey("Post", on_delete=models.DB_CASCADE)

class Profile(models.Model):
    user = models.OneToOneField("User", on_delete=models.DB_CASCADE)

Querying across relationships

# Filter comments by post fields (join via filter)
comments = await Comment.objects.filter(post_id__in=[1, 2, 3])

# Get all posts in a category
posts = await Post.objects.filter(category_id=category.id)