QGIS using project variables in filter queries

Question

In QGIS 2.14.11 LTR I have to apply the same (or similiar) filter query to various layers. I have the road name to filter for in a project variable and try to apply the following query:

"road_name" = @current_road

This works as expected in a 'select by expression', but returns 0 row as a filter query, where I obviously have to use

"road_name" = 'the road name'

As variables are obviously not evaluated in filter queries, is there a deeper reason for this that I don't see, and is there another way/syntax to use variables in filter queries?

Answer 1

ndawson is correct. You can use variables though in styling rules. This will help specifically if you only want to filter layers for showing or printing.

Answer 2

Layer filters don't use QGIS expressions, and are instead handed directly off to the underlying data provider. So, for instance, a filter on a postgis layer is passed to postgres to evaluate, and accordingly must use postgres syntax and functions. This means QGIS expressions functions and features like variables don't work in layer filters.