[Qgis-user] Altered behavior of Value Relation widget from QGIS ver. 3.38 to 3.40

[Bo Victor Thomsen]

To the list -

Please disregard my first post about this issue. I made an error by 
simplifying my example. The scenario is as follows:

I have *2* columns in a table - both with a "Value Relation" setup. The 
second column has furthermore a filter (/current_value('first_column') = 
"some_column_in_the_lookup_table"/ ) which decides the values in the 
value list for the second column based on the chosen value in the first 
column -  a "drill-down" effect.

In QGIS ver. 3.38 and prior versions -  you could change the behavior of 
the drop-down value list for the second column by checking/unchecking 
the "Allow NULL value" checkbox:

  * If "Allow NULL value" was checked, the chosen value in the second
    column would be "blank"/not chosen.
  * If "Allow NULL value" was /not /checked, the chosen value in the
    second would automatically be set to the first value in the filtered
    value list.

So you - as a QGIS project designer - could decide if the second column 
automatically got a value from the drop-down list for the second column 
when the user chose a value for the first column.

/However,/ in QGIS version 3.40 and onward, this behavior has changed. 
Regardless of the state of the "Allow NULL value" checkbox, the initial 
value of the widget for the second column is always "blank"/not chosen
(If manually activated, the drop down list for the second column 
contains the values derived from the filter expression. So the filter 
function is working)

Is this the new expected behavior or is it a regression bug ? And if it 
is the expected behavior: Can you somehow enable the old behavior ??

I have many (many, many...) QGIS projects depending on the old behavior.

Use case example: I have 2 columns in a table. The first column contains 
the Danish name for a tree species. The second column contains the Latin 
name for the tree species. So the value in the second column is 100% 
dependent on the user-chosen value in the first column. The user should 
not be bothered with selecting the Latin name in the second column from 
a value list with a single item.


Med venlig hilsen / Best regards

Bo Victor Thomsen

On 3/29/2025 10:47 AM, Bo Victor Thomsen wrote:
>
> Hi list -
>
> In QGIS ver. 3.38 and prior versions - using a "Value Relation" widget 
> in a attribute form - you could change the behavior of the drop-down 
> value list in the widget by checking/unchecking the "Allow NULL value" 
> checkbox:
>
>   * If "Allow NULL value" was checked, the chosen value  would be
>     "blank"/not chosen.
>   * If "Allow NULL value" was /not /checked, the chosen value would be
>     set to the first value in the value list.
>
> So you could decide if the field automatically got a value from the 
> drop-down list without any manual user action.
>
> /However,/ in QGIS version 3.40 and onward, this behavior has changed. 
> Regardless of the state of the "Allow NULL value" checkbox the initial 
> value of the widget is always "blank"/not chosen.
>
> Is this the new expected behavior or is it a regression bug ? And if 
> it is the expected behavior: Can you somehow enable the old behavior ??
>
> I have many (many, many...) QGIS projects depending on the old behavior.
>
>  --
>
> Med venlig hilsen / Best regards
>
> Bo Victor Thomsen
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