You can also use + and - prefix operators to indicate ordering:

# The following queries are equivalent:
Tweet.select().order_by(Tweet.created_date.desc())
Tweet.select().order_by(-Tweet.created_date)  # Note the "-" prefix.
# Similarly you can use "+" to indicate ascending order, though ascending
# is the default when no ordering is otherwise specified.

SELECT t1."id", t1."user_id", t1."message", t1."is_published", t1."created_date"
FROM "tweet" AS t1
INNER JOIN "user" AS t2
  ON t1."user_id" = t2."id"
ORDER BY t2."username", t1."created_date" DESC

When sorting on a calculated value, you can either include the necessary SQL expressions, or reference the alias assigned to the value. Here are two examples illustrating these methods:

query = (User
         .select(User.username, fn.COUNT(Tweet.id).alias('num_tweets'))
         .group_by(User.username)
         .order_by(fn.COUNT(Tweet.id).desc()))

Alternatively, you can reference the alias assigned to the calculated value in the select clause. This method has the benefit of being a bit easier to read. Note that we are not referring to the named alias directly, but are wrapping it using the helper:

ntweets = fn.COUNT(Tweet.id)
query = (User
         .select(User.username, ntweets.alias('num_tweets'))
         .join(Tweet, JOIN.LEFT_OUTER)
         .group_by(User.username)