Automatically fill in attributes of parent feature in QGIS

Question

I have a data table that holds different information about buildings in a GeoPackage. Different elements of the buildings can be entered, e.g. sometimes I only have the building as a whole, sometimes I have an entry for each room. Each entry has a "UUID" as a unique key. I use QGIS 3.16.3.

A simplified version of the table looks like this: enter image description here

You can download the simplified sample data here.

I added a field "uuid_parent" and created a recursive relation, which works fine, e.g. I can link a room to the building the room is in via the buildings "UUID". Now I would like to fill in all the information that necessarily are identical with the parent automatically, e.g. the address or the postal code.

The relation is defined like this:

enter image description here

I tried using a relation_aggregate() function as the field default in the layer properties on the field address like this:

relation_aggregate(
 relation:='entities_d_uuid_parent_entities_d_uuid',
 aggregate:='array_agg',
 expression:="address"
 )

Unfortunately I only get NULL-Values so far. I created a small sample data set, that shows, how it is supposed to look in the end. In this case, I had to type in the entries in the field "address" by hand, however:

enter image description here

Edit after hints by @pigreco and @Babel:

I edited the default expression into the following:

array_to_string(
 relation_aggregate(
 relation:='test_test',
 aggregate:='array_agg',
 expression:="address"
 )
 )

This shows me the correct values, but only in the preview of the expression and not in the attribute form or table when entering data.

• pigreco: the result of your expression is an array and therefore if the field is not correctly defined you will always get NULL
• C.-F. Vintar: Thanks for the clarification. Apparently this is not the only problem, however.
• Babel: Can you share your sample data set and the project? This would make it much easier to test - otherwise, one has to rebuild the whole project/data...
• C.-F. Vintar: Ok, edited the original again.
• Matt: You are sure the field you are calculating is a Text field with sufficient length to hold the string?

Answer 1

As @pigreco mentioned, the result of your expression is an array. To convert it to either a text or a number, you have different options (see documentation):

• To convert the array to a text (string), enclose your expression in an array_to_string( ) function.
• To get the the first element of the array, use array_first () - or array_last () for the last one.
• To get any element of the array, use array_get ( )[x] and replace x with the index of the element you want to get (0 for the first one).

• C.-F. Vintar: Thanks for the explanation. I tried all three variants, but I the result did not change. Two things appear as problems to me: 1) when entering data into the field "address" it jumps back to NULL, so I have to enter the data at least twice. 2) No matter what I enter as a parent feature, the field "address" remains NULL.
• Babel: You must provide more information. Can you provide sample data and show us how your attribute table looks like? Have you defined field-type and length correctly?
• C.-F. Vintar: I edited the original question. I hope that clarifies the question.