Show ValueMap values based on value of another field in QGIS widgets

Question

I'm applying a Value Map widget to two text fields in my layer. The two fields are as follows:

I have a hierarchy consisting of Landcover, and under that Landuse, where the color is code for the available option. For example, Residential is only a Built-up landcover, and Open only has Grassland and Woodland to chose from.

I want to apply a widget to the Lower field, where based upon the selected value of Upper, it display only the available choices for Lower

How can this be achieved with QGIS Attribute Form widgets, or other Layer Properties configurations?

Answer 1

The video below shows the result you want to get?

If so, follow these steps:

1. Create a table with unique values with three fields class, subclass, is_class_uniq (fill the first two fields, the third will be filled in automatically and must be boolean type, see step 3).
2. Create another table with two fields cla, sub_cla (it will have popup lists). Set up auto update (see how to do it here) for the field is_class_uniq using the formula:

if(
   "fid" = array_min(
                     array_agg("fid", group_by:="class")
                     ),
    true, false)

4. Fill in Filter expression box with these formulas below, cla field:

"is_class_uniq"

and sub_cla field:

current_value('cla') = attribute( $currentfeature, 'class') 

as shown in the pictures below:

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