Modelling UPSERT in SQLAlchemy (well actually it is not upsert but speed improvement is significant in comparison with simple session.merge)

sqlalchemy_upsert.py

# Note: it is a copy of great answer by "mgoldwasser" from Stackoverflow
# Check the original answer here: http://stackoverflow.com/a/26018934/1032439

# 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()