Reference selector with special constrainted by

My first post:
We all have sometimes the situation that we have to commit an object, because the retrieval of data for an reference selector will not work. The data depends on the selection of something else but the selection isn't of the same object. For example we have a form for an "Orderline" what contains the reference selector for the "Product". Here comes the tricky part, it may only be that "Product" that belongs to a "Category" selected in the "Order". The domain model can looks likes this.

In the modeler you can select the "Constraint by" property on a reference selector to let Mendix automatic constraint the data based on the associations. But in this case the "Order" is a new object and not committed to the database, so the results for the "Product" reference selector is empty. 

The reason for the missing data has to do how the constraint by function works. The constraint by function works for the first association, in this case "ODM.OrderLine_Order", and the other associations needs to be in the database, except the last one. The association "ODM.Order_Category" was not in the database and therefor the results were empty.

To solve this issue isn't to be committing the "Order" object before adding a new "OrderLine", but to use a datasource Microflow for the Reference Selector. In that Microflow you can retrieve the "Order" over association (from the memory of Mendix). Than retrieve all products that are attached to category based on the association of "ODM.Order_Category". You just don't have to retrieve the "Category" object first.