how to query a one to many relationship in sqlalchemy without duplicates - sqlalchemy

Question

I am trying to query the posts and i want to get all the comments too
this is my model for the parent and child
parent
class Post(Base):
__tablename__ = "posts"
id = Column(Integer, primary_key=True, index=True)
title = Column(String, nullable=False)
content = Column(String, nullable=False)
published = Column(Boolean, default=False)
created_at = Column(TIMESTAMP(timezone=True), nullable=False, default=text("NOW()"))
owner_id = Column(Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False)
comments = relationship("Comment", backref="post")
Child
class Comment(Base):
__tablename__ = "comments"
id = Column(Integer, primary_key=True, nullable=False)
email = Column(String, nullable=True)
name = Column(String, nullable=True)
content = Column(String, nullable=False) commented_at=Column(TIMESTAMP(timezone=True),nullable=False,server_default=text("NOW()"))
post_id = Column(Integer, ForeignKey("posts.id", ondelete="CASCADE"), nullable=False)
and im querying with join like this:
posts = db.query(models.Post, models.User, models.Comment).outerjoin(
models.User).order_by(models.Post.owner_id).outerjoin(models.Comment).order_by(models.Comment.post_id).all()
this is the result but here it is giving me 2 times the post with the id 1 for each comments if the post had 5 comments it will give me 5 times de same post
{
"Post": {
"title": "string",
"id": 1,
"published": false,
"owner_id": 1,
"content": "string",
"created_at": "2022-01-30T21:49:17.851582+02:00"
},
"User": {
"username": "TheBossmanLab",
"created_at": "2022-01-30T21:47:20.416480+02:00",
"email": "super_user#gmail.com",
"is_super_user": true,
"id": 1,
"password": "$2b$12$UEt0mzRp2qiLweJXwTztFuyWGcAfx6h5UFRE3avf3bt82wLzPZxTC",
"is_admin": true
},
"Comment": {
"commented_at": "2022-05-15T21:59:29.830714+02:00",
"id": 1,
"name": "666",
"email": "666#gmail.com",
"content": "granda post",
"post_id": 1
}
},
{
"Post": {
"title": "string",
"id": 1,
"published": false,
"owner_id": 1,
"content": "string",
"created_at": "2022-01-30T21:49:17.851582+02:00"
},
"User": {
"username": "TheBossmanLab",
"created_at": "2022-01-30T21:47:20.416480+02:00",
"email": "super_user#gmail.com",
"is_super_user": true,
"id": 1,
"password": "$2b$12$UEt0mzRp2qiLweJXwTztFuyWGcAfx6h5UFRE3avf3bt82wLzPZxTC",
"is_admin": true
},
"Comment": {
"commented_at": "2022-05-16T11:45:33.501141+02:00",
"id": 2,
"name": "666",
"email": "666#gmail.com",
"content": "nqowrbvowrb",
"post_id": 1
}
},
what i want as a result:
{
"Post": {
"title": "string",
"id": 1,
"published": false,
"owner_id": 1,
"content": "string",
"created_at": "2022-01-30T21:49:17.851582+02:00"
},
"User": {
"username": "TheBossmanLab",
"created_at": "2022-01-30T21:47:20.416480+02:00",
"email": "super_user#gmail.com",
"is_super_user": true,
"id": 1,
"password": "$2b$12$UEt0mzRp2qiLweJXwTztFuyWGcAfx6h5UFRE3avf3bt82wLzPZxTC",
"is_admin": true
},
"Comments": [{
"commented_at": "2022-05-15T21:59:29.830714+02:00",
"id": 1,
"name": "666",
"email": "666#gmail.com",
"content": "granda post",
"post_id": 1},
{"commented_at": "2022-05-16T11:45:33.501141+02:00",
"id": 2,
"name": "666",
"email": "666#gmail.com",
"content": "nqowrbvowrb",
"post_id": 1
}]
},
i think i over explained the situation but really im deseperate i think im blocked i cant think of any solution to this pleasse help me.

Answer 1

My understanding based on your question is that you want to query a list of posts which includes the information about the User and the Comments associated with the post.
If that is the case then instead of loading all three entities you could choose to create a relationship between the Post and User (referenced by the owner_id)
Since you're question does not offer details about whether a Post can exist without a User you can use the following query to fetch posts associated to a user.
from sqlalchemy.orm import joinedload, contains_eager
posts: Post = (
db.query(Post)
.join(Post.user, User.id == Post.owner_id)
.options(contains_eager(Post.user))
.options(joinedload(Post.comments))
.order_by(Post.id.desc())
.all()
)
If an association is created between Post and User then User.id == Post.owner_id becomes redundant and can be skipped.
The comments are simply eager loaded based on the relationship.
The join is only needed if there is no explicit relationships defined.
Both contains_eager and joinedload can be a bit tricky if you're new to sqlalchemy.
The TLDR version is that you'd use contains_eager where you've already instructed SqlAlchemy to load data with a join. In our case that would be the join(Post.user, User.id == Post.owner_id) while joinedload simply uses the defined relationships to eager load the association.
contains_eager
joinedload
Update
I see that you've mentioned the nullable constraint on the foreign_key for the Posts to User relationship. In that case this should be more than adequate.
posts: Post = (
db.query(Post)
.options(
joinedload(Post.user),
joinedload(Post.comments),
)
.order_by(Post.id.desc())
.all()
)