Hi AYBERK AKBALIK,
You can provide the restriction on 2 places.
1 – As soon as you select user run OC mf for the user and fetch only the project to which they are not assigned.
2 – After user and project selection and click on save
During save, get the select project list of the user – if the selection is matched to the selected input of the project on the page – then do not save and show error.
By the way if you mean Non-persistent object when referring to External object, then I suggest use persistent object!
Hope this helps!
You can solve this by using a retrieve from database, where you retrieve
[id = $Project] (where $Project stands for the project you currently have selected or is used as context)
[project_Person = $Person] (where $Person stands for the person you currently have selected or is used as context)
If a project is returned then the combination exists already. If not, then its a new combination.