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I am trying to autofill values of fields in a form when adding features to a database. When the point is added there are several fields that relate to polygons that the point sit within, such as within a county shape or a postcode district.

I would like to be able to set these fields automatically with the name of the intersecting polygon by running a spatial filter with the layers that contain the polygons and the point that is being added.

I have tried to do this by using the Relative Relation option in the fields widget when adding the point, but this still requires a click on the canvas in the correct polygon to fill the box. There are 3 fields that would need to be updated this way and if several points are being added it is a lot of extra mouse clicks that it would be preferable to avoid. This method could be error prone if the user clicks the wrong polygon. Also, if I looked at the info for an existing point, these fields would be empty and would need to be set again.

I thought that I may be able to set it using Value Relation, but I cannot figure out how to make this work. I assume that I would need to the geometry of the shape that I would be testing against the point, which I would not know beforehand.

I tried to script something to add as a Python Init function (after reading this article) , but I could not get the code to work properly in the Python Console. I would get the value for every point within the layer and I cannot see how to identify the point that I am attempting to add.

Is there a better way of doing this? Or is there some code that I could use that gets the coordinates of the point that is being added to do the intersection test?

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    I would think you would have success with PostGIS as the back-end, and using a trigger that would calculate the value with the spatial query you're using... Apr 4 '16 at 14:44
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    We have Oracle as our database so I could try and do it with triggers in Oracle, but I wanted to see if I could do it using QGIS.
    – Nicola
    Apr 4 '16 at 14:47
  • I had a similar question about how to simply fill in the time a feature was modified in QGIS, the answer was to use a trigger in the DB... Apr 4 '16 at 14:53
  • Ok I'll look into triggers in the database then. Thanks
    – Nicola
    Apr 4 '16 at 14:54
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After lots of trial and error I found a way of doing the spatial filter and then passing the value to a custom form, with a script running alongside.

The form takes dialog, the layerid and a featureid. I used featureid to identify the geometry of the point I wanted to test against polygons, obtained the geometry of the polygons to test against and if there was a match passed the value into the field in the form.

try:
    layer = QgsVectorLayer(uri.uri(), "TABLE_NAME", "oracle")
    feats = layer.getFeatures()
    for feat in feats:
        if featureid.geometry().within(feat.geometry()):
            textField.setText(feat["COLUMN"])
        else:
            pass

except Exception as e:
    msgBox = QMessageBox()
    msgBox.setWindowTitle("Error")
    msgBox.setText("Error: " + str(e))
    msgBox.exec_()

This saved having to create triggers and kept the logic contained in QGIS.

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