Including join features shape geometry in output feature class using Spatial Join in ArcPy?

Question

I am trying to join two feature classes together and also keep the spatial geometry of the join features. I want to keep the target and output geometry the same but also have a field like JoinSHAPE where I can access the geometry of the feature that it was joined with.

I have tried to add a field mapping for the join feature but an error occurs.

RuntimeError: FieldMap: Error in adding input field to field map

when I run the code below.

import arcpy

target = arcpy.GetParameter(0)
join_features = arcpy.GetParameter(1)
output =  r"S:\GIS TOOLS\Workbench.gdb\asdf"


fieldmappings = arcpy.FieldMappings()
fieldmappings.addTable(target)
fieldmappings.addTable(join_features)

shape = arcpy.FieldMap()
shape.addInputField(join_features, "SHAPE@")
fieldmappings.addFieldMap(shape)

arcpy.SpatialJoin_analysis(target, join_features, output, "#", "#", fieldmappings)

Answer 1

You can't join a geometry field. However, a nifty work-around could be populating a field with the geometry in WKT or JSON format:

json_field = 'JSON'
arcpy.AddField_management(join_features, json_field, 'TEXT', field_length=5000) # The length depends on how complex your geometries are.
with arcpy.da.UpdateCursor(join_features, ['SHAPE@JSON', json_field]) as cur:
    for row in cur:
        row[-1] = row[0]
        cur.updateRow(row)

...
shape.addInputField(join_features, json_field)
...

See here to learn about shape tokens.

Later on, when you want to read the joined geometry, you can use the AsShape tool:

with arcpy.da.SearchCursor(target, json_field) as cur:
    for row in cur:
        geometry = arcpy.AsShape(row[0], True)