Sorry because i'm a bit late ( work deadlines are struggling my time! :) ). I've made some different configurations and schema definitions... and i've noticed that it never updates a row if i set the datetime field as PK ( never! even if i set it as the only PK .. ). If i set composite PKs excluding any datetime column everything works fine ( it also works if i set a datetime as PK at the database side and excluding it at the schema on sqlalchemy side.. ). Sorry about the mess with the PKs between the former examples but i was only trying to say that changing the schema results that everything works fine...