Saving calculated fields to reuse in other QGIS projects

Question

I am using QGIS 3.4.

I want to make attribute tables with many calculated fields that link to styles, and reuse the same structure for many projects.

For example, when I draw a pipe (line vector), and enter attributes like diameter and flow, these should calculate a range of derived values that affect how the style is presented on the map.

In a new project when I make a new geopackage layer and apply the "Irrigation Pipe" style, how can I make sure the field calculations are in the table and auto-generated?

I could make a function for each type of field calculation, but would I have to manually load the functions to each field every time I made a new attribute table?

QGIS novice here, it's been 20 years since I did lots of GIS!

Answer 1

The Attributes Form settings are saved in the QGIS style of the layer. There you can define an expression to default values for new and updated rows.

• https://docs.qgis.org/3.4/en/docs/user_manual/working_with_vector/vector_properties.html#default-values

All the field widget (with the default expression) setting will be copied to a layer with the same name and type of field, when the QGIS style is loaded to that layer.

I have tried to refer to other values of the same row without problem with an expression like this:

attribute($currentfeature,'flow') / attribute($currentfeature, 'diameter')

Note that the expression will be evaluated if a row is created or updated, so if you want to auto populate existing rows you will need to do some trick: something like create a new dumb field and then update it from the Field Calculator with any random value, or write some script that update all the rows for you.

A last note: you will find a problem if you want to evaluate the default value on the provider side and the layer is not a Postgre SQL table. But if you are OK with QGIS evaluating the default value, the field widget expression works very well.