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Python

Posted by Paul Finch
Paul Finch
5,484 Points

A peewee query

Here is my data model

from peewee import *

database = SqliteDatabase("sounds.sqlite")


class Publisher(Model):
    name = CharField(unique=True)

    class Meta:
        database = database


class Artist(Model):
    name = CharField(unique=True)

    class Meta:
        database = database


class Album(Model):
    artist = ForeignKeyField(Artist, related_name="artist_name")
    title = CharField()
    release_date = DateTimeField()
    publisher = ForeignKeyField(Publisher, related_name="publisher_name")
    media_type = CharField()

    class Meta:
        database = database

and here is my query

from models import *


query = (Album
         .select()
         .join(Publisher)
         .join(Artist)
         .where(Album.title == 'Once Upon A Time'))

for disk in query:
    print disk.title, disk.publisher.name, disk.artist.name

Trying to return the title along with the publisher and the artist. This throws the following error

File "C:/Users/*********/PycharmProjects/PeeWeeTest/core.py", line 25, in <module>
    .join(Artist)
  File "C:\Python27\lib\site-packages\peewee.py", line 383, in inner
    func(clone, *args, **kwargs)
  File "C:\Python27\lib\site-packages\peewee.py", line 2739, in join
    raise ValueError('A join condition must be specified.')
ValueError: A join condition must be specified.

How do i create this sort of query across the three tables?

Brian Anstett
Brian Anstett
5,831 Points

Hey Paul, I had this EXACT same problem. I found the answer on the peewee doc pages. I looks like when you use the join() function, peewee will use the last joined table as the new source table for the next join. So for example, after you join Album with Publisher using the foreign key, an error occurs with the next join between Publisher and Artist because there there is no relationship between Publisher and Artist.

The solution is to use the switch() function to switch back to the original table after the first join with Publish, before the second join with Artist.

So the query you would want would be... query = (Album .select() .join(Publisher) .switch(Album) .join(Artist) .where(Album.title == 'Once Upon A Time'))

http://docs.peewee-orm.com/en/latest/peewee/querying.html?highlight=peewee.join#joining-on-multiple-tables.