Limiting list values based on previous field in QGIS Forms?

Question

Looking for assistance to set up appropriate configuration to enable drop-down lists values to be filtered based on a previous value.

Basically, I have a 3-tiered categorisation structure to one of our GIS layers - Class > Type > Sub Type.

Once Class has been selected I would like the Type list to be filtered to only values relevant for that class and so on for Sub-Type.

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I have turned this into Code Value list attribute tables and loaded inot QGIS as attribute tables -

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I have used these tables as Value Relation lists in the widget properties

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The drop down lists work fine without a filter. But as soon as I try adding a filter with an expression, there is nothing in my drop down... If I simply type "CLASS_ID" = 1 ... I do limit the list to only those for Class = 1. The expression on it's own (attribute($currentfeature, 'LU_CLASS') correctly populates the integer values if I use it to populate a virtual field in the expression builder...

Can anyone provide any guidance as to why this doesn't work in the widget filter?

I have also (briefly) tried to set up a relation in the project properties... but this feels like I am going about it the wrong way... the records aren't a child or related to this other table - I simply want to use them to populate drop-down lists.

Am I just missing a simple step?

• Matthias Kuhn: Would this core feature already be what you want? gis.stackexchange.com/questions/156039/…
• Jamie: Hi Matthias, this other question was also posted by me (after I had little feedback on this one and did a bit more experimentation myself) - it kind-of works, but requires far more setup and overcomplicates what should be relatively simple. As I understand it, the "relation reference" is for true parent-child dependencies, such as a pole or feature that has multiple connections to it. The suggestion below of a "$currentformfeature" expression sounds great to me, and like it would make the above situation work.

Answer 1

$currentfeature refers to the related layer (Type_Values in your example) so the expression always return false and all values are filtered out.

See: https://lists.osgeo.org/pipermail/qgis-developer/2015-November/040382.html

• Jamie: This sounds like a wonderful suggestion! Would be very keen to see this implemented so that the above workflow example would work :)
• Luffydude: ermm am i missing something? nothing comes out of that link

Answer 2

I've just published a plugin that allows for drill-down and other complex filtering of values in a form, have a look:

http://www.itopen.it/a-new-qgis-plugin-allows-dynamic-filtering-of-values-in-forms/

Answer 3

I needed something similar and in my case I made it work changing your expression:

"CLASS_ID" = attribute( $currentfeature, 'LU_CLASS')

for

"CLASS_ID" = current_value('LU_CLASS')

Brief explanation:

As @elpaso66 noticed $currentfeature refers to the related layer (Type_Values in the example above) and we need a reference to the element being edited at the time, so we have current_value('field') function that allows us to select the value of a field in the element that we are editing.

This way we can filter the key of the related layer (Type_Values in the example above) with the value of the fileld we want to use as filter (LU_CLASS in the example above)

• root676: Welcome to GIS.SE! Could you please present your answer in more detail so that other users can follow your solution approach more easily? Thanks!