Filtering in overlay_nearest

Question

I am using QGIS 3.24,

I have 2 polylines layers:

• First one is composed of 2 rivers, crossing. The two of them are named 1 and 2
• Second one is composed of parallel lines form this rivers. 5 and 6

Starting from my parallel layer (red), i would like to find the nearest entity in my layer river (blue), BUT with a filter. Indeed, i want to join parallel line with the same river they're from. So I DON'T want my parallel line 5 to be joined with my river 1, but i want it to be joined with my river 2.

I've tried this, with field_calculator, from my layer parallel:

array_to_string(overlay_nearest('river_layer',id,filter:=river_name='river_name',limit:=1))

id is id from river_layer

But nothing happens! If i write it like this:

array_to_string(overlay_nearest('river_layer',id,filter:=river_name='1',limit:=1))

It's working, but it's only pointing to entity from river 1, ofc.

I've read https://github.com/qgis/QGIS/issues/43146 but I don't get this formula: \'layer2\'? Why \? And this [0]? I don't get it neither.

---

I think I've misled the first answerer. My river is 'segmented', so it may cause some trouble with that answer, even if it could work:

On this picture, lines 31, 32, 33 and 34 are from the same river (It's already stored in my tables, river and parallel lines). I have 4 parallel lines, and they are segmented too (I marked them with point at the middle of them). We can imagine a part of the attribute table like this:

If I put a buffer on my river (I can even do it BEFORE river is segmented), I fear that some parallel segmented lines might be associated with the wrong river, like on my picture, which shows some errors:

Maybe I could create a virtual layer, with something like this, considering P_para_line as my point layer with points in the middle of parallel segmented lines:

SELECT
    ST_startpoint(ST_ShortestLine(p.geometry, r.geometry)),
    r.id AS id_of_the_segmented_portion_of_river,
    ROUND(ST_Length(ST_ShortestLine(r.geometry, p.geometry)), 6) AS distance
FROM
    P_para_line AS p, river_layer AS r
WHERE
    p.river_name = r.river_name

After this, I get way too many points on the same coordinate! I wish I could keep only the point with the shortest distance. I can suppress other points manually, but I think i can change my virtual layer query.

Answer 1

Create a buffer around the river layer (containing 1 and 2) with a distance slightly larger then the distance to the second line layer (containing 5 and 6).

Then check within which of the buffers the second line is and get the value of an attribute (like id) by its field name, e.g. using FIeld calculator.

Use this expression:

array_max (
    array_foreach(
        overlay_nearest (
            'buffer',  -- change the layer name
            $id,
            limit:=2  -- change the number, depending how many rivers/parallel lines you have
        ),
        if (
            within (
                $geometry,
                geometry(
                    get_feature_by_id (
                        'buffer',  -- change the layer name
                        @element
                    )
                )
            ),
            attribute (
                get_feature_by_id (
                    'buffer',  -- change the layer name
                    @element
                ),
                'id'  -- change the name of attribute whose value you want to get
            ),''
        )
    )
)

Red line is labeled with the expression above, getting the value of the id of from the layer river (blue):