QGIS Processing Modeller - Select date by expression

Question

I have a data file that contains current meter data for multiple days.

I am trying to use the Processing Modeller to extract a single days worth of data for further manipulation, using the Extract by Expression algorithm.

My input data layer has the date in it, with the format dd/mm/yy (i.e 01/12/19)

I've tested my selection expression using Select by expression in QGIS as per below, which correctly selects 1 days worth of data.

Building my model, I select the input later using a vector feature parameter definition. I then used String parameter definition to input the required date '25/12/19' and the date field "%Date"

I entered the below expression into the Processing Modeller to match the Select by Expression that I tested ealier.

When I run the model, nothing is extracted to the result layer.

I'm sure it's probably a syntax problem, but can't figure it out.

Answer 1

I suspect its checking the literal value of the DateField Input against the literal value of the EnterDate input. In other words, its not extracting the value of the field named by the value of DateField. This looks like too many levels of indirection.

You probably need to use the attribute function to get the value of an attribute in a layer by a variable holding the name of the field. After a few tests I've got this to work:

 attribute(@datefield) = @enterdate

BUT you don't put the field name in quotes in the Input - just %Date, not "%Date" as you had. (Note I did everything in lowercase but that's not totally necessary)

To test it, I used the Field Calculator algorithm to create a new layer so I could see what the expression engine was computing for each part of the expression. After about 50 iterations of various attempts I think I've got it...

Answer 2

I use QGIS 3.16.10. The function "Select by expression" works very well outside the modeler. But, it didn't work in modeler. I ended up switching to a more straightforward tool -- the "Select by attributes".