blog.notmyidea.org/content/code/2023-10-18-sqlalchemy-many-to-one-queries.md

title	headline	tags
Using DISTINCT in Parent-Child Relationships	How to get parent and most-recent child in a one-to-many relationship	SQL, SQLAlchemy, Python

Let's say you have a model defined like this, with a Parent and a Child table:

class Parent(Base):
    __tablename__ = "parent"
    id: Mapped[int] = mapped_column(primary_key=True)

    childs: Mapped[List["Child"]] = relationship(back_populates="parent")


class Child(Base):
    __tablename__ = "child"
    id: Mapped[int] = mapped_column(primary_key=True)
    parent_id: Mapped[int] = mapped_column(ForeignKey("parent.id"))
    parent: Mapped["Parent"] = relationship(back_populates="children")

    born_at: Mapped[datetime] = mapped_column()

I've tried many ways, with complex subqueries and the like, before finding out the DISTINCT SQL statement.

So, if you want to retrieve the parent with it's more recent child, you can do it like this:

results = (
    db.query(Parent, Child)
    .join(Child)
    .distinct(Parent.id)
    .order_by(Parent.id, desc(Child.born_at))
    .all()
)