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I am looking for a way to implement conditional value mapping in QGIS. This means that the values available in attribute number 2 depend on the value provided in attribute 1.

Example:

If the user enters "agriculture" in attribute 1, for attribute 2 only "corn" or "wheat" will be available.

Instead, if the user enters "forest" in attribute 1, for attribute 2 only "pines" or "olives" will be available.

And so on...

Apparently this is not doable just using the QGIS widget expression builder, but I am not sure. Here's a screencast to help visualize. The problem is that if for "class" I pick other than "Agri_Field", for the "color_aggri" attribute the same possible values appear.

enter image description here

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  • This is plugin/standalone application or you want to do this with standard Qgis tools (like subtypes in ArcGiS)?
    – dmh126
    Commented Apr 23, 2015 at 11:17
  • I wish to do this with standard tools - no plugin development
    – Lisdengard
    Commented Apr 23, 2015 at 11:56

3 Answers 3

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The thread is quite old, but still one of the first answers to the problem found on Google. Therefore a small update: For QGIS 3.X the plugin from Lisdengard's answer does not exist anymore. From 3.4 onward this feature is now part of the core functions and can be found in Properties > Attribute Forms > Value Relation. See https://north-road.com/drill-down-cascading-forms/ and https://www.northrivergeographic.com/archives/qgis-3-2-drive-down-lists-aka-value-relation-widget for a short introduction.

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A similar function has been introduced recently

It requires the possible values to be grouped on the related layer and is slightly more involved in setup.

If you have a layer

Plots

  • Name
  • class (foreign key to Classes:pk)

And a layer

Classes

  • pk
  • Type [Agriculture/Forest]
  • Plant [Pines/Olives/Corn/Wheat]

You will need to add a new relation in Project Properties=>Relations that defines the foreign key from Plots:class to Classes:pk.

Then you can define a relation reference widget on Plots:class with pre-filters checked (this option is new and was introduced AFTER 2.8. Until the next release, get a nightly/master build!) and check "Type" as pre-filter.

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  • Thanks Mathias! Though not exactly what I would like it (I would prefer a recursive solution), I tried it and it's definitely a valid work around! Again, thanks a lot!
    – Lisdengard
    Commented Apr 23, 2015 at 19:50
  • Can you explain what you mean with recursive?
    – Matthias Kuhn
    Commented Apr 24, 2015 at 8:02
  • Maybe not the best word, but what I meant is that it would be nice if no extra table would be needed - conditioning "on fly" attribute 2 by what was entered first in attribute 1. This something usually required for mobile GIS apps, but I think it has it's space on desktop GIS too.
    – Lisdengard
    Commented Apr 24, 2015 at 11:48
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This issue is now easy to tackle:

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

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