Ui policy or something else?

[B_pppppp]

I need to check if a field has no value available to choose from. If the field has no value, another fields needs to be mandatory. Can I do this in ui policy? How?

[B_pppppp]

not only empty... the field is a reference to a table. There is a reference qualifier on it. If there is no available values to be chosen in the field, then another field should be filled in. 

Let's say that my field is referenced to the sys_user table. And in my field I can choose an user from a list of users that have "S" in their name. In the sys_user table there is no user that satisfy the requirement so I do not have what to choose. In this case, another field should be mandatory

[vamsi krishna8]

What if there is a Value and the user don't choose the value from it ?

[B_pppppp]

then it should be an error

[vamsi krishna8]

Instead of making such Complex , i recommend , if the field value is Blank(either no choice to choose or the user did not select) then make the other field Mandatory using UI Policy .
If possible post the requirement/Use case clearly so that some one can provide the Solution !