Build a dictionary from an object's explicitly declared fields

My table definition:

class AppProperties(base):
    __tablename__ = "app_properties"
    key = Column(String, primary_key=True, unique=True)
    value = Column(String)

Code for updating an app property:

session = Session()
query = session.query(AppProperties).filter(AppProperties.key == "memory")
new_setting = AppProperties(key="memory", value="1 GB")
query.update(new_setting)

The line query.update(new_setting) fails because the update method requires an iterable. If I use query.update(vars(new_setting)), it includes some extra values, which are not there in the underlying table and hence fails.

How can I build this dictionary: {"key": "memory", "value": "1 GB"} using the object new_setting, so that I can call the update method using this dictionary?

For example:

query.update({"key": "memory", "value": "1 GB"})

Because I already have everything in new_setting, I just need to convert it into a dictionary with only those keys which are explicitly declared in the class definition of AppProperties, without inheriting the fields from the base class or any other scope.