QGIS – Filtering a ‘Value Relation’ to Show Only Intersecting Features

qgisvaluerelation

I am trying to model the 'splicing' of cables (Table 1) together at a splice point (Table 2).

Table 1 – "Cables" (linestring)
Table 2 – "Nodes" (point)
Table 3 – "splices" (geomless table with int fields for ID of "Nodes", and "Cables" a&b)

I have created the third table ("splices") to do this by using a value relation to both Nodes & Cables ('a' & 'b' for the two ends of the cable).

When creating a new record in "splices", once "Node" is selected I would like to only return nearby "Cables".

To filter the Cables I have used:

intersects($geometry, buffer(geometry(get_feature_by_id('[qgis node layer]','7067')), 0.000005))

This works, but as you can see, I am referring to Node id (7067) statically. How can I instead use the Node id of the selected Node?

I wondered if I need to use current_parent_value(field_name) to find the Node id, but can't seem to make that work.

Best Answer

You can use current_value('field_name') to access the unsaved value of field_name in the current form. In this case, it's the node id.

Adapt the following expression in your splice layer Value Relation widget under Filter Expression for both Cable A and Cable B fields:

intersects($geometry,
    buffer(geometry(get_feature('node_layer','node_id',current_value('node'))),0.5))

Or, if you want the Cable B field to automatically bring up the Cable B value based on the Cable A value, use the above expression for Cable A, and the following one for Cable B - preferably make it read only if you want to avoid accidentally changing it.

"cable_a_id" = current_value('cable_a')

Demonstration of using the first expression for Cable A and second expression for Cable B.

Widget settings in splice layer for Cable A field

Widget settings in splice layer for Cable B field

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qgis – Setting Value Relation Relative to Other Field in QGIS 3

We can use a Virtual Field to do this. Create Field 2 as a virtual field and with an expression.
In the 'Maple' example above we want to turn a string into another based on the Field1 value. For this we can use a CASE Expression such as:

CASE 
WHEN "FIELD1" = 'Acer platanoides' THEN 'Maple'
WHEN "FIELD1" = 'Quercus alba' THEN 'White Oak'
END

As values are added or edited to Field1 Field2 will update once a save is made. We can add as many WHEN checks to the CASE as we need.

In my example below Field1 has the name 'HubDist' & we are dealing with numbers not strings

QGIS Form – Setting Relation Value in QGIS Form

Keep the "old" field with the old observers, then create a new field with value relation that you use for new oberservs: those you take with value relation from the table. So you have a clean dataset and don't have to mix old and new observers with the risk of altering data.

You can define a constraint in the "new" field so that if the "old" field already contains a name, it should stay empty.

To combine both old and new observer values, create a 3rd field as virtual filed that dynamically updates and that combines both fields, like:

if (length ("name_old") > 0, "name_old",  represent_value ("name_new"))

This field is what you're looking for as it combines bot manually entered values as well as the names from the value relation.

Best Answer

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qgis – Setting Value Relation Relative to Other Field in QGIS 3

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