Filter expression not working for Value Relation widget in 2024.1.0

[raherin]

When using a filter expression on a value relations widget, the app will add all values that meet the filter expression when only one value is selected in the drop-down menu while filling out the form. This can be reproduced in the example project support/form_setup.

To reproduce the issue:

1. Have a survey layer with a Value Relation widget and set a filter expression based on the value of another field:

2. The expected behaviour, which is reproduced in QGIS, is that when selecting a plant type in the form, you will be able to select a plant of that type in the form. In this case, 'Vegetable' was selected as the plant type and 'Carrot' as the plant species:

3. When filling out the form in the app, all of the species are saved to the field in the form. See video below:

Screen_Recording_20240514_164208.mp4

[uclaros]

This weird behavior is actually caused by the project not being set up properly:

While the value relation for field type is properly set:

• Key column is Code, -> stores values [A, B, C ...]
• Value column is Description, -> displays values [Fruit, Vegetable, Flower ...]

The value relation for sub-type is wrong:

• filter expression is OK, only shows entries where Code matches the type's selected Key.
• Value column is Species, -> displays values [Carrot, apple ...]
• ❌ Key column is Class, -> stores values [A, B, C ...] which do not correspond to a single Value entry

Setting the Key column to either fid or Species solves it, as there is now a 1-1 connection to the Value values.

So the question is: what should be displayed when the selected Key corresponds to multiple Values, and should we somehow warn users about it?
QGIS handles it by displaying the first matching value.
I believe our approach is more correct, we display a concatenated string of all corresponding values.

[wonder-sk]

Note: there is a ticket for the plugin to detect such problems: MerginMaps/qgis-plugin#317

[tomasMizera]

Ping @raherin