[QGIS-Developer] Python example with aggregate calculation

[Neumann, Andreas]

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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