QGIS Form – Setting Relation Value in QGIS Form

attribute-tableqgisvaluerelation

I'm building a form to collect observation data of species. I have a layer called "observation" (point) with a field called "observer", it is the name of who sees the species. Secondly, I have a table with all the employees of the company.

In my form, I use a relation value for the field "observer" with my table, so that I have a drop-down menu with all the employees. It works perfectly.

The problem is in my layer "observation", I have many other observers, and they disappeared when I used the relation value, cause they are not in the list.

How can I have a drop-down menu with my employees, for new inputs through the form, but still let me have other "old" records with different employees?

Best Answer

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.

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QGIS Custom Widgets – Sorting Drop-Down Lists in Value Relation Widget

Due to the fact that I am poorly versed in German - I created a separate data set in English, which should be understandable to all readers of the site.

In order to sort the data in the QGIS drop-down list according to a manually defined order, you need to have the following fields in the tables.

Table 1:

table1_sort - manual sorting of rows.
table1_type - groups.

Table 2:

table2_sort - manual sorting of rows.
table2_type - groups.
table2_objects - objects.
table2_sort - The field that contains the same values as the table1_sort field.

New data will be entered in Table 3. It should contain the following fields:

types - groups.
objects - objects.

Now go to Table 3 field paramaters and set the Widget type to Value Relation.

Now select the types field and set the following parameters for it:

Layer -> Table 1
Key column -> table1_sort
Value column -> table1_type

Now select the objects field and set the following parameters for it:

Layer -> Table 2
Key column -> table2_sort
Value column -> table2_objects
Filter expression -> current_value('types') = "table1_sort"

Now you may try to enter your data with the Drop-down list (with manual sorting):

<== (click to watch)

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

Best Answer

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QGIS Custom Widgets – Sorting Drop-Down Lists in Value Relation Widget

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

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