QGIS Action to update several fields

Question

I would like to update one ore more fields of my attribute table on demand (f.e. using a QGIS Action) with one simple click.
For now I have to open attribute table or the layer properties, start field calculator, choose the field I want to update, type or paste my expression and run it for every field I need to update.
I tried to use the model builder but I found no way, to simply update a field within the layer without creating a new layer.
Virtual Fields are not helping me in this way, because the recalculation will be time consuming (complex aggregate expression).

TeddyTedTed · Answer

You can do this using virtual fields. In the field calculator check the Create virtual field button: 
When you add or edit a record or close and reopen the attribute table this will recaulcuate update on the fly.
For example if I create a virtual field for a polygon layer with the expression $area then when I create a new polygon it will automatically populate that field. If I change the size of the polygon then it will update.
You can also change the expression later by going to Layer Properties > Fields and edit the expression in there.

QGIS Action to update several fields

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