PyQGIS – Selecting Point with Maximum Value for Each Polygon

Question

I have a point layer with two fields named "id" and "price", and a polygon layer with a field named "code_zone".

I need to get, for each polygon, the "id" of the point that has the highest value of "price", and this maximum value of the "price".

This is the beginning of my code:

for feature in layer_point.getFeatures():

   point_price = feature.attribute('price')
   point_geom = feature.geometry()

   for feat in layer_polygon.getFeatures():

      polygon_codeZone = feat.attribute('code_zone')
      polygon_maxValue = feat.attribute('max_value')
      polygon_geom = feat.geometry()

      if polygon_geom.contains(point_geom):
             ...

I tried with maximumValue(), but I get only the maximum value of the whole point layer and not the maximum value per polygon. What I am doing wrong?

As soon as I get these maximum values, I could fill them in my polygon layer with polygon_layer.changeAttributeValue() in the "max_value" field.

Answer 1

The user of a SQL query is appropriate for your problem.

I created 2 tables: one with 2 polygons and another with 5 points.

Then I go to the database manager > Virtual layers > and open a SQL query window

I execute the following request :

--First part : For each polygon, the prices of all the points that are contained.
-- An identifier (fid) is generated for each polygon in descending order of price.

WITH all_price AS
 (SELECT row_number() over(partition by poly.id_poly  ORDER BY point.price DESC) as fid, 
 poly.id_poly, point.id, point.price
 FROM poly, point
 WHERE St_contains(poly.geometry,point.geometry)
 ORDER BY poly.id_poly, point.price)

 --When fid is equal to 1, it means that it is the highest price.
 SELECT all_price.id_poly, all_price.id, all_price.price
 FROM all_price 
 WHERE fid = 1

Result of the temporary table all_price on the left and the final result on the right. If you don't understand the request, I can give you more explanations.