Is there a way in QGIS to connect two fields belonging to two different layers of the same type?

For example in my case I have a linear "father" layer containing all the information of the pipelines of my network, from this layer I extracted another "son" layer containing the same elements but with different fields, except for some that are identical (for example the lengths of the pipes).

I would like to make sure that when I update the lengths contained in the "father" layer, the lengths in the "son" layer are updated automatically. Can you help me?

I attach a screen of the two layers, in which I would like to make sure that if I update the "L_LUN (m)" field of the "father" layer, also the "Length" field of the "son" layer is updated automatically.

enter image description here

  • 1
    Did you consider virtual layers?
    – Babel
    May 27, 2020 at 17:56
  • Thanks for your help, I was in fact trying to figure out how to connect the fields together using a virtual field, but I don't know what functions I must use. However, using a virtual field does not modify the original shapefile, while I would like the shapefile to be modified. May 27, 2020 at 18:07
  • 1
    Normally what you are doing would be easily achieved with a trigger if you were using a database (eg sqlite, postgres). However I don't think QGIS has an out of the box way to automatically update a child layer every time the parent layer changes. The only option might be to use python code in Actions
    – she_weeds
    May 28, 2020 at 1:26

1 Answer 1


You can use a virtual field in your son-layer which takes the length from the father layer. So when the length in the father will be changed, the virtual field in the son-layer will reflect the changes:


this expression will take the value from field "length" of the father layer with the same id: "id_son"=attribute(@parent,'id_father')) You have to adjust the fieldnames to your layer, like id_son = Name and id_father = ID_COND.

enter image description here

  • Thank you very much for your answer, I tried the function but unfortunately the resulting field in the child layer (which I called "Paullo_link") is empty, maybe I'm wrong: I created a new virtual field in the child layer that I called LUNGHEZZA and I inserted this expression: aggregate( layer:='CONDOTTE EDITATE', aggregate:='max', expression:="L_LUN (m)", filter:="Name"=attribute(@parent,'ID_COND')) did I enter wrong? Thanks for your help! May 28, 2020 at 9:52
  • Should I create a field that has the same name as that of the parent layer, that is "L_LUN (m)" in my case? I wish that if I update the field of the parent layer "L_LUN (m)", it should automatically update the "Length" field in the child layer, being: Parent layer: 'CONDOTTE EDITATE' with filter id: "ID_COND" Child layer: 'Paullo_link' with filter id: "Name" May 28, 2020 at 10:04
  • Ciao Enrico, in your case it should be something like aggregate( layer:='CONDOTTE EDITATE', aggregate:='max', expression:="L_LUN (m)", filter:="Name"=attribute(@parent,'ID_COND'))
    – eurojam
    May 28, 2020 at 11:51
  • it's exactly what i tried but unfortunately it doesn't work .. Grazie per l'aiuto! May 28, 2020 at 13:08

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