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I've got a line layer called "Water Lines - Proposed" that has multiple line features. Each feature has a 'Type of Feature' field (multiple features may share the same value), a 'Name' field (each feature has a unique name), and a 'Length' field, which is calculated.

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I've got a second layer with a value relation field allowing me to select multiple features from the "Water Lines - Proposed" layer by 'Name' from a filtered list with [Spillway (sill)] as the 'Type of Feature'. I would like to then calculate the total length of all features that have been selected in this list ('Spillway (sill) Length').

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The following expression worked for getting the length of a single feature, but I cannot figure out how to modify it now that I have allowed for multiple selection.

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You can use the relation_aggregate function in the field calculator on the parent layer like so:

relation_aggregate(
    relation:='my_relation_name',         -- the name of the relation
    aggregate:='sum',                     -- the type of aggregation
    expression:="Length (ft)"             -- the field in the child layer to aggregate
)

The child layer:

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The parent layer (with the calculated field):

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  • Thanks @matt - I'm confused about what to put for the "my_relation_name" part of the expression? The list of spillway names that I can select from in the "Associated Spillway (sill) Feature" prompt is provided by a value relation widget in the form, with "Water Lines - Proposed" selected as the layer and "Name" selected as both the Key column and Value column.
    – Brightside
    Commented Nov 11, 2022 at 0:29
  • Ah, I didn't realise you were using a widget. The expression works when you set up the relation through the project properties. Try the expression in this answer instead. It doesn't use a relation per se, but it matches features using a common field.
    – Matt
    Commented Nov 11, 2022 at 7:06

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