Filter expression builder in QGIS 2.8 allows us to define custom functions using python. I'm intrested would it be possible to define a function which returns a value of a field in an unsaved form that is currently being filled. I would like to use this output in a filter expression for value relation.
Possibility of reading the contents of unsaved fields is mentioned in theese two posts, but i lack the knowledge of python to adapt it to purpose mentioned above:
If this is not overly complicated i would be very gratefull for an example of code i could addapt to my purpose.