SQLAlchemy many to one and selection of the last/first child

content/Technologie/.ztr-directory

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

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+---
+title: Using DISTINCT in Parent-Child Relationships
+headline: How to get parent and most-recent child in a one-to-many relationship
+tags: SQL, SQLAlchemy, Python
+---
+
+Let's say you have a model defined like this, with a Parent and a Child table:
+
+```python
+
+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](https://www.sqlitetutorial.net/sqlite-distinct/).
+
+So, if you want to retrieve the parent with it's more recent child, you can do it like this:
+
+```python
+
+results = (
+    db.query(Parent, Child)
+    .join(Child)
+    .distinct(Parent.id)
+    .order_by(Parent.id, desc(Child.born_at))
+    .all()
+)
+```