Hi,
Finally, with the support from Matthias, I got a working example of my
custom Python expression:
@qgsfunction(args='auto', group='Custom', usesgeometry=False)
def get_ews_anlage_nr(gemeinde, feature, parent):
AnlageLayer =
QgsMapLayerRegistry.instance().mapLayersByName('Anlage')[0]
aggParams = QgsAggregateCalculator.AggregateParameters()
aggParams.filter = 'substr("anlage_nr",1,2) = \''+gemeinde+'\''
maxVal =
AnlageLayer.aggregate(QgsAggregateCalculator.Max,'to_int(substr("anlage_nr",4,4))',aggParams)[0]
return maxVal
When I define the QgsAggregateCalculator.AggregateParameters() upfront
separately it works fine. First I was trying to set the parameter
directly within AnlageLayer.aggregate() but failed to find the direct
syntax of the QgsAggregateCalculator.AggregateParameters() parameters.
There are hardly any examples out there. But hey - finally it works!
Have a nice weekend!
Andreas
On 2017-05-05 15:55, Neumann, Andreas wrote:
Hi,
I have a basic example using aggregates:
----------------------
@qgsfunction(args='auto', group='Custom', usesgeometry=False)
def get_ews_anlage_nr(gemeinde, feature, parent):
AnlageLayer =
QgsMapLayerRegistry.instance().mapLayersByName('Anlage')[0]
maxVal = AnlageLayer.aggregate(4,'to_int(substr("anlage_nr",4,4))')[0]
return maxVal
-----------------------
But I still have two questions:
1. I still struggle to define a filter. I can't find the correct syntax
how to define that filter string.
2. the first parameter "4" stands for "Max" aggregate (according to
http://qgis-python.kartoza.com/docs/core/other/QgsAggregateCalculator.html#class-qgsaggregatecalculator
). Could this code be written more "readable" by specifying "Max"
instead of 4?
Thanks,
Andreas
On 2017-05-05 14:31, Neumann, Andreas wrote:
Hi,
Does someone have an example how to use QgsVectorLayer.aggregate()
from Python?
How would the syntax for the parameters in the
QgsAggregateCalculator.calculate() method look like in Python?
Thanks,
Andreas
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