How to increase a counter in SQLAlchemy

Question

Suppose I have table tags which has a field count that indicates how many items have been tagged with the given tag.

How do I

Answer 1

I got here following a tutorial for FastAPI, SQLAlchemy and databases (async). After various tries I finally got it working with following code in my crud.py / control layer.

async def inc_counter():
    query = counters.select(counters.c.id==1)
    current_val = await database.fetch_val(query=query, column=1)
    query = counters.update().where(counters.c.id==1).values(counter=current_val+1).returning(counters.c.counter)
    return await database.execute(query=query)

My table looks like this:

counters = Table('counters', metadata,
    Column('id', Integer, primary_key=True),
    Column('counter', Integer)
)

Answer 2

If you are using the SQL layer, then you can use arbitrary SQL expressions in the update statement:

conn.execute(tags.update(tags.c.tag_id == 5).values(count=tags.c.count + 1))

The ORM Query object also has an update method:

session.query(Tag).filter_by(tag_id=5).update({'count': Tag.count + 1})

The ORM version is smart enough to also update the count attribute on the object itself if it's in the session.

Answer 3

If you have something like:

mytable = Table('mytable', db.metadata,
    Column('id', db.Integer, primary_key=True),
    Column('counter', db.Integer)
)

You can increment fields like this:

m = mytable.query.first()
m.counter = mytable.c.counter + 1

Or, if you have some mapped Models, you can write alternatively:

m = Model.query.first()
m.counter = Model.counter + 1

Both versions will return the sql statement you have asked for. But if you don't include the column and just write m.counter += 1, then the new value would be calculated in Python (and race conditions are likely to happen). So always include a column as shown in the two examples above in such counter queries.