Hello, I am working on a test project, the popular invoice system, but I am noticing some failures, I do not know if I am doing the wrong relationship, but when adding a subform, it is giving me an error when saving, individually the forms save the information, but already combined it does not save.
Below I share the screenshots, both of how I made the relationship of the tables and the error and the project in production.
Very good, hopefully it can be solved soon 
I have been reviewing the code that it generates, as I see if the code of $ rec_id is generated for the detail table of product_sale (Relationship between sales and products) to generate the invoice. As it is a one-to-many relationship, it seems that when the sale_id object is passed it is not being sent as an array, I have already tried several things and apparently it would be to modify the code generator to solve that detail.
Maybe they can solve it soon or you will have a solution to the possible logic error that you are having
The detail master already works in version 3.9