Undocumented mergeRule in FieldMapping

Variable80 — Tue, 27 Jun 2023 22:00:26 GMT

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?

Re: Undocumented mergeRule in FieldMapping

SamSzotkowski — Tue, 08 Aug 2023 15:43:16 GMT

Awesome find! If you're not comfortable using the undocumented merge rule in your production code, maybe you could use a Join merge rule into a text field, then follow up your spatial join with a calculate field or update cursor to parse out the number of unique values in each string (split it into a list on the join delimiter, then convert the list to a set and find the set's length).

topic Re: Undocumented mergeRule in FieldMapping in Geoprocessing Questions

Undocumented mergeRule in FieldMapping

Re: Undocumented mergeRule in FieldMapping