SQLAlchemy DetachedInstanceError

Why This Happens

SQLAlchemy uses lazy loading by default. When you close the session then access a relationship, SQLAlchemy cannot load data because the session is gone.

The Problem

def get_user(user_id):
    with Session(engine) as session:
        user = session.get(User, user_id)
    return user

user = get_user(1)
print(user.posts)  # DetachedInstanceError

The Fix

from sqlalchemy.orm import joinedload

def get_user(user_id):
    with Session(engine) as session:
        user = session.get(User, user_id,
            options=[joinedload(User.posts)])
    return user

Step-by-Step Fix

1. 1

   Use eager loading

   Add joinedload() or subqueryload() to the query.

2. 2

   Access data in session

   Access lazy-loaded relationships while session is open.

3. 3

   Keep session scope wider

   Keep session open for the entire operation.