How to compute the distance from the current polygon centroid to another in QGIS field calculator?

Question

There is layer with polygons. I want to save in new field distance of every polygon's centroid to centroid of one particular polygon.

To do this I should write some expression like this:

distance(centroid($geometry), get_feature(6679)) # 6679 is feature id of some polygon

According to weak documentation get_feature returns the first feature of a layer matching a given attribute value. Writing it like this

get_feature('id', 6679)

results into "Parsing Error: Function is called with wrong number of arguments".

Who knows the right way to get geometry of some polygon with given feature id?

Answer 1

The function you are looking for is actually getFeature (at least in QGIS 2.8). It takes 3 arguments (hence your "Parsing Error") : the name of the layer, the name of the field, and the value of the field. So it would be something like getFeature( 'your_layer', 'id', 6679 ).

Additionnal point : you actually have to use geometry(getFeature( ... )) in your expression, because distance takes geometry arguments. In the end, you will have :

distance( centroid($geometry), geometry(getFeature('your_layer', 'id', 6679)) )

Answer 2

Your answer, actually helped, but I didn't find a way to get feature by feature id. So I created new field ID containing feature id, expression is simple:

@row_number 

And then solved my task

distance(centroid($geometry), centroid(geometry(getFeature(@layer_name, 'ID', 6609))))

Thanks!