python - SQLAlchemy - performing a bulk upsert (if exists, update, else insert) in postgresql

Question

I am trying to write a bulk upsert in python using the SQLAlchemy module (not in SQL!).

I am getting the following error on a SQLAlchemy add:

sqlalchemy.exc.IntegrityError: (IntegrityError) duplicate key value violates unique constraint "posts_pkey"
DETAIL:  Key (id)=(TEST1234) already exists.

I have a table called posts with a primary key on the id column.

In this example, I already have a row in the db with id=TEST1234. When I attempt to db.session.add() a new posts object with the id set to TEST1234, I get the error above. I was under the impression that if the primary key already exists, the record would get updated.

How can I upsert with Flask-SQLAlchemy based on primary key alone? Is there a simple solution?

If there is not, I can always check for and delete any record with a matching id, and then insert the new record, but that seems expensive for my situation, where I do not expect many updates.

Answer 1

There is an upsert-esque operation in SQLAlchemy:

db.session.merge()

After I found this command, I was able to perform upserts, but it is worth mentioning that this operation is slow for a bulk "upsert".

The alternative is to get a list of the primary keys you would like to upsert, and query the database for any matching ids:

# Imagine that post1, post5, and post1000 are posts objects with ids 1, 5 and 1000 respectively
# The goal is to "upsert" these posts.
# we initialize a dict which maps id to the post object

my_new_posts = {1: post1, 5: post5, 1000: post1000} 

for each in posts.query.filter(posts.id.in_(my_new_posts.keys())).all():
    # Only merge those posts which already exist in the database
    db.session.merge(my_new_posts.pop(each.id))

# Only add those posts which did not exist in the database 
db.session.add_all(my_new_posts.values())

# Now we commit our modifications (merges) and inserts (adds) to the database!
db.session.commit()