Question

I have two tables Items and ItemDescriptions

class Item(Base):
    __tablename__ = "item"
    id = Column(Integer, primary_key=True)
    description_id = Column(Integer, ForeignKey('item_description.id'))

class ItemDescription(Base):
   __tablename__ = "item_description"
   id = Column(Integer, primary_key=True)

Given a list of ItemDescriptions I want a list of Items such that there is one item of each ItemDescription id. I don’t care which item.

[Edited for clarity]

Given this list of item and descriptions:

Item, Description
1   , 1
2   , 1
3   , 1
4   , 2
5   , 2
6   , 3
7   , 3
8   , 3

I want a query that will return something like:

Item, Description
2   , 1
4   , 2
7   , 3   

I’m having trouble working out the sub-queries etc.

Thank you for your help

Answer 1

I am a big fan of column_property. Here’s a way to do what you want with column_property:

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class Item(Base):
    __tablename__ = 'item'

    id = Column(Integer, primary_key=True)
    description_id = Column(Integer, ForeignKey('item_description.id'))

class ItemDescription(Base):
    __tablename__ = 'item_description'

    id = Column(Integer, primary_key=True)

    any_item_id = column_property(
        select(
            [Item.id],
            id == Item.description_id,
            limit=1,
        ).label('any_item_id'),
        deferred=True,
    )

e = create_engine('sqlite://', echo=True)
Base.metadata.create_all(e)

s = Session(e)

descriptions = [
    ItemDescription(id=1),
    ItemDescription(id=2),
    ItemDescription(id=3),
]

s.add_all(descriptions)

items = [
    Item(id=1, description_id=1),
    Item(id=2, description_id=1),
    Item(id=3, description_id=1),
    Item(id=4, description_id=2),
    Item(id=5, description_id=2),
    Item(id=6, description_id=3),
    Item(id=7, description_id=3),
    Item(id=8, description_id=3),
]

s.add_all(items)

query = s.query(ItemDescription).options(undefer('any_item_id'))
for description in query:
    print description.any_item_id, description.id

# alternative way without using column_property
query = s.query(
    select(
        [Item.id],
        ItemDescription.id == Item.description_id,
        limit=1,
    ).label('any_item_id'),
    ItemDescription,
)
for item_id, description in query:
    print item_id, description.id