I have two point datasets; Person (in red) and Connections (in blue).
They have matching columns Person_UID.
I'm trying to connect the red points to all the blue points that have matching Person_UIDs.
So far I am only able to make one connection for each UID in'Connections' the point that was first digitized.
Code:
make_line( $geometry, geometry(get_feature('Connections', 'Person_UID', "Person_UID")) )
I'm using QGIS 3.12.0
Screen grab attached.
Your expression is correct but you are using the wrong layer.
To create a group of lines that connects all the Person points with the Connections points you need to change the order of the layers.
Instead using Person Layer, create a line with Geometry generator in the Connections layer with the expression
make_line( $geometry, geometry(get_feature('Person', 'Person_UID', "Person_UID")) )
It will create the line that you want.
Just wanted to add another answer in case anyone has a similar situation but wants to use the Person layer for some reason or wants a many-to-many join.
You need to use aggregate() - which returns multiple matches from one point returned as a multigeometry (one to many join), instead of get_feature() which will only ever return one feature each time (one to one or many to one join).
However, the expression that I got to work was a lot more complex than I thought. You can't use make_line() to join a single point (from the Persons layer) to multipoints (matching points in Connections layer). So you have to convert the Connections multipoints to an array of single points and then make a line from each Person point to it. I did this using generate_series() to create an array of number of points and looped through the multipoints using that and array_foreach().
collect_geometries(array_foreach(
generate_series(1,num_geometries(
aggregate('connections','collect',$geometry,
"Person_UID"=attribute(@parent,'Person_UID')))),
make_line($geometry,geometry_n(
aggregate('connections','collect',$geometry,
"Person_UID"=attribute(@parent,'Person_UID'))
,@element))))
The bonus is that this works for many-to-many joins as well, where get_feature() won't work no matter which layer you use it on.
EDIT: The above works if your Persons layer is a Point geometry (not Multipoint). Your Connections layer can be a Point or Multipoint geometry.
If your Persons layer is a Multipoint geometry then make_line() using $geometry won't work - you need to convert your Persons layer to single point either by creating a new layer, or using this even lengthier expression to loop through the multipoint geometries in each Person feature.
Would be nice to have a "promote to single parts" expression in the QGIS expression builder...
collect_geometries(array_foreach(generate_series(1,num_geometries(aggregate('connections','collect',$geometry,"Person_UID"=attribute(@parent,'Person_UID')))),make_line(array_foreach(generate_series(1,num_geometries($geometry)),geometry_n($geometry,@element)),geometry_n(aggregate('connections','collect',$geometry,"Person_UID"=attribute(@parent,'Person_UID')),@element))))
