Qgis 2.8: How to define a custom function that returns value of a field in an unsaved

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Filter expression builder in QGIS 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: QGIS forms: How to limit available values in a dropdown field according to value selected in a 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.

Much obliged,



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