Xtykutl

I have the following code

def update(project_id, code, description):



    if project_id is None:

        raise exception



    with session_handler() as session:

        project = session.query(Project).filter_by(project_id=project_id).first()

        project_upd = session.query(Project).filter_by(project_id=project_id)



    if project is None:

        raise ProjectDoesntExist(f"Project {project_id} does not exist.")

    data = _build_update_data(code, description)



    if  not data:

        raise ValueError("No code or description provided")



    project_upd.update(data)

So if I replace project_upd.update(data) with project.update(data) it gives the following error

Attribute Error: Project object has no attribute update.

How I can use only one variable?

Though the Project model is omitted, it is clear that it has no method update() – which is the norm. The difference is that project is bound to a Project object, and project_upd to a Query object. In other words the former represents a single row in the table mapped to an object, while the latter represents a query against that table.

Your options are:

1. Issue an UPDATE statement only and check if it matched any rows, raise if not.


2. Fetch the row / object for update, check if it existed and raise if not, do the update.

I assume that session_handler() commits, if no exception was raised. If that is not the case, add an explicit session.commit() as necessary.

1. Single UPDATE statement

def update(project_id, code, description):

    if project_id is None:

        raise exception



    data = _build_update_data(code, description)



    if not data:

        raise ValueError("No code or description provided")



    with session_handler() as session:

        row_count = session.query(Project).

            filter_by(project_id=project_id).

            update(data)



        if not row_count:

            raise ProjectDoesntExist(f"Project {project_id} does not exist.")

2. Fetch and update

def update(project_id, code, description):

    if project_id is None:

        raise exception



    data = _build_update_data(code, description)



    if not data:

        raise ValueError("No code or description provided")



    with session_handler() as session:

        # Fetch FOR UPDATE so that no concurrent updates may proceed in between

        # getting the `Project` instance and actually updating it.

        project = session.query(Project).

            filter_by(project_id=project_id).

            with_for_update().

            first()



        if project is None:

            raise ProjectDoesntExist(f"Project {project_id} does not exist.")



        for attr, val in data.items():

            setattr(project, attr, val)

Locking with FOR UPDATE might be a bit unnecessary in this case, since the new values don't seem to depend on previous state of the object. Still, it is something to keep in mind.