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.

(see: Limiting available values in dropdown field of QGIS form according to value selected in previous field?)

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:

QGIS Value Map Edit Widget with properties dependent on another field

How to automatically populate fields instantly?

If this is not overly complicated i would be very gratefull for an example of code i could addapt to my purpose.


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