check if query exists using peewee

Question

I am using the Peewee library in Python and I want to check if a query exists. I do not want to create a record if it doesn't exist, so I don't want to use get_or_create. There must be a better solution than to use try/except with get but I don't see anything. Please let me know if there is a better way. Thanks.

Answer 1

If you just need to check existence use the accepted answer.

If you are going to use the record if it exists you can make use of Model.get_or_none() as this removes the need to use a try/catch and will not create a record if the record doesn't exist.

class User(peewee.Model):
    username = peewee.CharField(unique=True)

user = User.get_or_none(username='charlie')
if user is not None:
    # found user, do something with it
    pass

Answer 2

Alternatively, if you want to check if e.g. some other table refers this record, you can use WHERE EXISTS (subquery) clause. It is not supported natively by PeeWee, but it can be easily constructed:

subquery = Child.select(Param('1')).where(Child.parent == Parent.id)
parents_with_children = Parent.select().where(
    Clause(SQL('EXISTS'), subquery))

It is equivalent to the following SQL:

SELECT * FROM parent
WHERE EXISTS (SELECT 1 FROM child
              WHERE child.parent_id = parent.id);

Here I used SELECT 1 for subquery to avoid fetching unneeded information (like child.id). Not sure if such optimization is actually required.

UPD (Feb 2022)

After more than 5 years of peewee evolution, it looks like the Clause class is gone.

The following code may work (I didn't have a chance to test it though):

subquery = Child.select(Param('1')).where(Child.parent == Parent.id)
parents_with_children = Parent.select().where(
    NodeList((SQL('EXISTS'), subquery)))