Automatically copy across data from one layer to another where the field name is the same in QGIS

Question

Layer A contains data in "Field A", Layer B also contains a field named "Field A", can I automatically transfer the data in "Field A" from Layer A to Layer B?

I used to use RefFunctions but I cannot figure out if the new overlay functions allow this type of transfer.

Answer 1

Based on spatial relationship

To do it based on spatial relationship, you can use one of the overlay functions, e.g. using one of the following expressions. The first two only work if the two lines are exactly the same (version 3 even considers line direction) - in many cases, even if they should be so, in fact they are not. In such cases, you might want to use the last version.

• overlay_within('LayerA',"name_A")[0]

• overlay_contains('LayerA',"name_A")[0]

• overlay_equals('lineA',name)[0]: this works only if the lines have the same direction (line A to B). For a line B to A, it will return NULL

• overlay_nearest('lineA',name)[0]: this works if lines or not completely equal: it takes the nearest line of Layer_A to the one of Layer_B

Based on common id

To copy the content of field name_A from LayerA to a field in LayerB, where you use a common id-Filed - where the value of the id from LayerA (id_A) correspods to the id value from LayerB (id_B) - use this expression:

attribute (
    get_feature_by_id (
        'LayerA',
        "id_B"
    ),
    'name_A'
)