How to do an upsert with SqlAlchemy?

Question

I have a record that I want to exist in the database if it is not there, and if it is there already (primary key exists) I want the fields to be updated to the current state

Answer 1

I use a "look before you leap" approach:

# first get the object from the database if it exists
# we're guaranteed to only get one or zero results
# because we're filtering by primary key
switch_command = session.query(Switch_Command).\
    filter(Switch_Command.switch_id == switch.id).\
    filter(Switch_Command.command_id == command.id).first()

# If we didn't get anything, make one
if not switch_command:
    switch_command = Switch_Command(switch_id=switch.id, command_id=command.id)

# update the stuff we care about
switch_command.output = 'Hooray!'
switch_command.lastseen = datetime.datetime.utcnow()

session.add(switch_command)
# This will generate either an INSERT or UPDATE
# depending on whether we have a new object or not
session.commit()

The advantage is that this is db-neutral and I think it's clear to read. The disadvantage is that there's a potential race condition in a scenario like the following:

• we query the db for a switch_command and don't find one
• we create a switch_command
• another process or thread creates a switch_command with the same primary key as ours
• we try to commit our switch_command