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I have a point layer (Dormouse) linked to a child geometry-less table (DormouseSurveyData) with a 1-n relationship. I am trying to set up the symbology of the point layer to change color dependent on if there is a value in the 'Presence' field of my child table.

I am aware that I could either join the tables or create a virtual layer to achieve this but I need the symbology to update as the records are updated so that when data is input in the field the symbology will dynamically update accordingly.

I have tried many different variations but the ones i thought most likely to work are below:

case
    when dbquery('DormouseSurveyData','Presence','Tube_ID = attribute($currentfeature,'id')')
        then color_rgb(0,200,0)
    
    when 'DormouseSurveyData_2acaff7f_d2b5_47a2_875c_0859987007d7.Tube_ID' =  'Dormouse.id' and 'DormouseSurveyData_2acaff7f_d2b5_47a2_875c_0859987007d7.Presence' = "False"
        then color_rgb(200,0,0)
    
    else color_rgb(0,0,200)
end

case
    when dbvalue('DormouseSurveyData','Presence','Tube_ID', 'ID')
        then color_rgb(0,200,0)
    when dbvalue('DormouseSurveyData','Presence','Tube_ID', 'ID')
        then color_rgb(200,0,0)
    else color_rgb(0,0,200)
end

The relationship links Dormouse.id to DormouseSurveyData.Tube_ID

Below are snippets of the two tables. Both with testing data.

When I run either expression in the data override for the fill color of my symbology i just get the 'else' result (ie blue).

Dormouse Table

DormouseSurveyData Table

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  • How did you realize the link? Can you provide example data? What happens when you try your expressions?
    – Erik
    Sep 16, 2021 at 12:29

2 Answers 2

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Have you tried relation_aggregate function for classification of the layer?

E.g.

relation_aggregate(
relation:='relationdomousexblaablaablaa21321144124',aggregate:='max',expression:="precense") 

relation_aggregate function brings the id or what ever field you need "Up" to your visualization layer and then you can classify your data based on the other 1:N table values.

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Based on the answer provided by Sanna Jokela, I created the code below which works perfectly for my purposes and can be easily expanded upon:

case 
when relation_aggregate('DormouseSu_Tube_ID_Dormouse_c_id', 'count', "presence" = 1) = 0 then color_rgb(0,0,200)
else color_rgb(0,200,0)
end

For anyone else wanting to use this, the field 'Presence' is just a confirmation field so only contains '1' for yes or '0' for no. The code above counts up the number of times the child features have a value of '1'. If that is >0 then I know there was confirmed presence on my point so it can be colored accordingly. If not, then the else statement kicks in and puts the default color.

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