QGIS - python - read value from current form

Question

I would like to implement a function to check that there are no features already inserted with the same value in a certain field.

This is the function that I implemented as Action but during editing [%id_lavoro%] always seems to be empty

from qgis.utils import iface
from qgis.PyQt.QtWidgets import *
msg = QMessageBox()
l = iface.activeLayer()
expr = QgsExpression( "\"id_lavoro\"=[%id_lavoro%]")
it = l.getFeatures(QgsFeatureRequest(expr))
ids = str([i.id() for i in it])
msg.setText(ids)
msg.show()

QGIS has a function (current_value) but I don't know how to use it in Python code

enter image description here

• Giovanni: I continue to do research but it seems there is no way to return value of a field in an unsaved form.......

Answer 1

From what I understand, you don't need any action here. You can just set a unique constraint on your field.

See: https://www.youtube.com/watch?v=gG0Yg5JRDP8 for how to setup this on your form.

• Giovanni: My control (Action) is only for warning, it must not be a constraint
• YoLecomte: If you only check the "unique" checkbox, it won't act as a constraint but as a warning. Maybe you need to don't check "enforce unique constraint"?.
• Giovanni: What I would like is to show a list of the other rows (list of rows IDs) that refer to the same "id_lavoro" so the user can check if it is an error or not