Aggregating records

Suppose you have some users and want to get a list of them along with the count of tweets in each.

query = (User
         .select(User, fn.Count(Tweet.id).alias('count'))
         .join(Tweet, JOIN.LEFT_OUTER)
         .group_by(User))

The resulting query will return User objects with all their normal attributes plus an additional attribute count which will contain the count of tweets for each user. We use a left outer join to include users who have no tweets.

Let’s assume you have a tagging application and want to find tags that have a certain number of related objects. For this example we’ll use some different models in a many-to-many configuration:

class Photo(Model):
    image = CharField()
class Tag(Model):
    name = CharField()
class PhotoTag(Model):
    photo = ForeignKeyField(Photo)
    tag = ForeignKeyField(Tag)

Now say we want to find tags that have at least 5 photos associated with them:

query = (Tag
         .select()
         .join(PhotoTag)
         .join(Photo)
         .group_by(Tag)
         .having(fn.Count(Photo.id) > 5))

This query is equivalent to the following SQL:

SELECT t1."id", t1."name"
FROM "tag" AS t1
INNER JOIN "phototag" AS t2 ON t1."id" = t2."tag_id"
INNER JOIN "photo" AS t3 ON t2."photo_id" = t3."id"
GROUP BY t1."id", t1."name"
HAVING Count(t3."id") > 5

Suppose we want to grab the associated count and store it on the tag:

query = (Tag
         .select(Tag, fn.Count(Photo.id).alias('count'))
         .join(PhotoTag)
         .join(Photo)
         .group_by(Tag)
         .having(fn.Count(Photo.id) > 5))