To add on to this, I discovered why it was trying to insert NULL for relation fields. I did the select before setting my relations. And it wasn't enough to have a separate model load the related table. It has to be loaded in the QSqlRelationTableModel for it to work. So the very original comment here (sorry if this pings you) turned out to be 100% correct. I'm just also posting this here for future reference @JonB said in QSqlRecord forces null in composite primary key on QSqlTableModel::insertRecord: Isn't that actually a consequence of you using a QSqlRelationalTableModel? Isn't it that the Qt side expects to see in the related FoodData table/model an entry for the value of your food column in FoodDay table/model equal to it? You have "broken the contract" you establish with QSqlRelationalTableModel that Qt will see in-memory models maintaining the relationship? It is not good enough if you say the row does exist at the backend database side, if you want to use QSqlRelationalTableModel (you don't have to, it's only for value lookup) you must have the FK model/table up-to-date in memory.