Drill down form does not update as in QGIS

[cdreiser]

I have a very similar setup of three connected widgets as shown in the attached gif.
In QGIS this works as expected, i.e. a change in field "Land use" results in blanking both fields "Type" and "Plant type".

This differs to the gif-animation whereby a change of "Land use" results in the selection of the first "Type" option of the list and consequently the selection of its appropriate "Plant type" options.

Blanking both "Type" and "Plant type" is of course preferable, in order to force a user decision instead of proposing one.

In QWC, however, a change in "Land use" changes "Type" to its first option on the list, but does NOT change "Plant type"
With the result that selecting an unsuitable "Plant type" is possible, which should not be the case.

[manisandro]

This would be best investigated directly on the affected setup. I suggest either debugging around [1] and inside the EditComboField component [2] and ideally submitting a PR with a fix, or feel free to reach out for commercial support [3] if you need external help with debugging.

• [1]

  qwc2/components/QtDesignerForm.jsx

  Line 408 in e180e0c

  let filterExpr = null;

• [2] https://github.com/qgis/qwc2/blob/e180e0cf3ae1a71e91383d525899c368a26bdf65/components/EditComboField.jsx
• [3] https://qwc-services.github.io/master/#support

[cdreiser]

Thanks for the hints, manisandro.
I managed to find my work around without digging into the code. Not perfekt, but for the time being.

My work around (I explain this with the field names of the animation):
the filter expression for "Plant type" must not only check the present content of "Type" but also of "Land use". In QGIS this is not required: it seems to "drill down" through all combo boxes that are somehow connected by a filter expression.

The work around, however, does not solve two problems

a) the check-status "checked" of a "Plant type" option is kept even if "Land use" or "Type" has been changed. Resulting in a possible combination of cereals, fruits and vegetables (which should not be possible). I shall try to resolve this through an on change event in JS.

b) I require that all fields (Land use, Type and Plant type) are NOT NULL. I have set constraints accordingly. This works fine for the multi-select ComboBox on INSERT, however it does not on UPDATE of a record. Reason seems that even if you uncheck all of the multi-select options, the empty curly brackets of the array remain in the table record and this seems not to be regarded as equal to NULL.

I do not close this issue as it is not resolved, just worked around.

[cdreiser]

In order to solve the two remaining problems mentioned above, I use as workaround an UPDATE row-trigger that fires BEFORE.

Any inconsistency of the selected combination of Type and Plant type results in an exception dialog that describes the problem and avoids writing the result to the database tabel as long as any of those incionsistencies prevail.

I know, resolving the issue in the backend via trigger is not the ideal approach, but I am not (yet) ready to dig into ReactJS to resolve this in the frontend via events. After all my approach does the trick and saves me keeping my deadline.

Given for the sake of explanation only one table t_landuse_type_planttype that contains all possible combinations, e.g.
id, landuse, type, planttype
1, 'Farmland', 'Cereals', 'Wheat'
2, 'Farmland', 'Cereals', 'Rye'
3, 'Farmland', 'Cereals', 'Barley'
etc.

... the Postgresql trigger function is as follows (modified to match the example shown in the gif-animation above)

DECLARE
  arr text[];
  cleaned_arr text[];
  invalid_elems text[];

BEGIN

  -- Check, whether NEW.planttype is NULL, empty or '{}'
  IF NEW.planttype IS NULL OR trim(NEW.planttype) = '' OR trim(NEW.planttype) = '{}' THEN
    RAISE EXCEPTION '
	
    Note: 
    At least one plant types nedds to be selected. 
    Please correct!';
  END IF;

  -- NEW.planttype is string of format '{"Apple","Orange","Carrot"}'
  -- it needs to be convertet to an array
  arr := string_to_array(trim(both '{}' from NEW.planttype), '","');

  -- get rid of the double quotes
  SELECT array_agg(trim(both '"' from elem)) INTO cleaned_arr
  FROM unnest(arr) AS elem;

  -- collect invalid elements 
  SELECT array_agg(elem) INTO invalid_elems
  FROM unnest(cleaned_arr) AS elem
  WHERE NOT EXISTS (
    SELECT 1 FROM t_landuse_type_planttype t
    WHERE t.type = NEW.type AND t.planttype = elem
  );

  IF invalid_elems IS NOT NULL THEN
    -- set planttype to NULL to avoid inconsistencies
    NEW.planttype := NULL;
    RAISE EXCEPTION '
	
    Note: 
    Plant type(s) % are invalid for the selected Type "%".
    Please unselect invalid Plant type for selected Type!',
    array_to_string(invalid_elems, ', '), NEW.type;
  END IF;

  RETURN NEW;
END;