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Undocumented mergeRule in FieldMapping

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07-13-2023 09:43 AM
Variable
New Contributor

I've made an interesting finding with SpatialJoin_analysis's field mapping. It appears that there are some undocumented merge rules that produce correct output.

My scenario is as follows. I have one layer that contains points with several fields. The second layer contains a grid that I would like to aggregate the points from the first layer into. To accomplish this I am calling the SpatialJoin_analysis operation:

 

arcpy.SpatialJoin_analysis(
    layer_grid,
    curr_layer,
    new_layer_path,
    join_operation="JOIN_ONE_TO_ONE",
    join_type="KEEP_COMMON",
    match_option="INTERSECT",
    field_mapping=get_field_mapping(curr_layer))

 

where the output of the get_field_mapping(curr_layer) method returns a valid field mapping.

 

def get_field_mapping(layer):
    """Creates a mapping between selected fields from the origin GDB to the
    output GDB. The output fields will aggregate multiple entries either by
    a sum or count operation.

    """
    fm_value_16 = arcpy.FieldMap()
    fm_vreg = arcpy.FieldMap()
    intsct_mapping = arcpy.FieldMappings()

    fm_value_16.addInputField(layer, "PRICE_2016_ADJUSTED")
    fm_value_16.mergeRule = "Sum"
    fm_out_val_16 = arcpy.Field()
    fm_out_val_16.name = "Value_2016_Sum"
    fm_out_val_16.type = "Double"
    fm_value_16.outputField = fm_out_val_16

    fm_vreg.addInputField(layer, "V_REG_NUMBER")
    fm_vreg.mergeRule = "Unique"
    fm_out_vreg = arcpy.Field()
    fm_out_vreg.name = "VRN_Unique_Count"
    fm_out_vreg.type = "Integer"
    fm_vreg.outputField = fm_out_vreg

    intsct_mapping.addFieldMap(fm_value_16)
    intsct_mapping.addFieldMap(fm_vreg)

    return intsct_mapping

 

Each point in the first layer has two fields, PRICE_2016_ADJUSTED and V_REG_NUMBER, that I want merged. For PRICE_2016_ADJUSTED I want its values to be summed, while for V_REG_NUMBER I want to calculate the number of unique values. Essentially, I want functionality similar to the Sum and Unique rules from (https://pro.arcgis.com/en/pro-app/2.9/tool-reference/analysis/summary-statistics.htm). However, according to the documentation for fieldMap (https://pro.arcgis.com/en/pro-app/latest/arcpy/classes/fieldmap.htm) I'm restricted to only using "Sum" as a valid mergeRule.

Yet, this line (fm_vreg.mergeRule = "Unique") does in fact produce the number of unique entries in the aggregated point! Is this behaviour intentional and if so, can this be used in production code?

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