Spatial Join Yielding NULL values in ArcPy?
I have recently had some troubles getting my spatial join to work, running into similar issues as you were. I am not familiar with the field mapping enough to know if the way you are field mapping is possible or not. All I know is, this is the way I got field mapping (with a merge rule on multiple fields) to work for me.
# Copy features to a new layer
arcpy.CopyFeatures_management(target,outputfeatureAPT,config_keyword="#",spatial_grid_1="0",spatial_grid_2="0",spatial_grid_3="0")
arcpy.CopyFeatures_management(joinFeatureSTUD,outputfeatureSTUD,config_keyword="#",spatial_grid_1="0",spatial_grid_2="0",spatial_grid_3="0")
# Delete fields in copy features before you spatial join
arcpy.DeleteField_management(outputfeatureAPT,drop_field="APT;ADDRESS;PA;TYPE;STATUS;LIU;MRU;UNITS;CLASS;LOW_INCOME;APP_NUM;ELEM;INT_;MID;HIGH;ELEM_STUD;INT_STUD;MID_STUD;HIGH_STUD;TOT_STUD;ELEM_YLD;INT_YLD;MID_YLD;HIGH_YLD;TOT_YLD;LAST_EDIT;NOTES")
# The following inputs are layers or table views: "stud_0915_1"
arcpy.DeleteField_management(outputfeatureSTUD,drop_field="Loc_name;Status;Score;Match_type;X;Y;Match_addr;Side;Pct_along;ARC_Street;ARC_Zip;ARC_City;ARC_Postal;ISD;STUD_ID;NAME;F_NAME;L_NAME;ADDRESS;ZIP;CITY;CAMPUS_ID;SCHOOL;GRADE;ETHNICITY;ED;BIL;PA;RESIDE;CLASS;SUB;EE_PK;EE;PK;KG;GRD1;GRD2;GRD3;GRD4;GRD5;GRD6;GRD7;GRD8;GRD9;GRD10;GRD11;GRD12;AMER_IND;ASIAN;BLACK;HISPANIC;WHITE;PACIFIC;MULTI;BIL_EE;BIL_PK;BIL_KG;BIL_GRD1;BIL_GRD2;BIL_GRD3;BIL_GRD4;BIL_GRD5;BIL_GRD6;BIL_ELEM;BIL_INT;BIL_MID;BIL_HIGH;ED_ELEM;ED_INT;ED_MID;ED_HIGH;AMER_IND_ELEM;AMER_IND_INT;AMER_IND_MID;AMER_IND_HIGH;ASIAN_ELEM;ASIAN_INT;ASIAN_MID;ASIAN_HIGH;BLACK_ELEM;BLACK_INT;BLACK_MID;BLACK_HIGH;HISPANIC_ELEM;HISPANIC_INT;HISPANIC_MID;HISPANIC_HIGH;WHITE_ELEM;WHITE_INT;WHITE_MID;WHITE_HIGH;PACIFIC_ELEM;PACIFIC_INT;PACIFIC_MID;PACIFIC_HIGH;MULTI_ELEM;MULTI_INT;MULTI_MID;MULTI_HIGH;SF;MF;EXEMPT_65;RENTER")
# Create a new fieldmappings and add the two input feature classes.
fms = arcpy.FieldMappings()
fms.addTable(outputfeatureAPT)
fms.addTable(outputfeatureSTUD)
ELEMSumIndex = fms.findFieldMapIndex("ELEM")
INT_SumIndex = fms.findFieldMapIndex("INT_")
MIDSumIndex = fms.findFieldMapIndex("MID")
HIGHSumIndex = fms.findFieldMapIndex("HIGH")
TOT_STUDSumIndex = fms.findFieldMapIndex("TOT_STUD")
# Create the required FieldMap and FieldMappings objects
fm_apt_poly_APT_ID = arcpy.FieldMap()
fm_stud_ELEM = fms.getFieldMap(ELEMSumIndex)
fm_stud_INT_ = fms.getFieldMap(INT_SumIndex)
fm_stud_MID = fms.getFieldMap(MIDSumIndex)
fm_stud_HIGH = fms.getFieldMap(HIGHSumIndex)
fm_stud_TOT_STUD = fms.getFieldMap(TOT_STUDSumIndex)
fms = arcpy.FieldMappings()
# Create the required FieldMap and FieldMappings objects
fm_apt_poly_APT_ID = arcpy.FieldMap()
fm_stud_ELEM = arcpy.FieldMap()
fm_stud_INT_ = arcpy.FieldMap()
fm_stud_MID = arcpy.FieldMap()
fm_stud_HIGH = arcpy.FieldMap()
fm_stud_TOT_STUD = arcpy.FieldMap()
fms = arcpy.FieldMappings()
# Get the field names for both original files
apt_poly_APT_ID = "APT_ID"
stud_ELEM = "ELEM"
stud_INT_ = "INT_"
stud_MID = "MID"
stud_HIGH = "HIGH"
stud_TOT_STUD = "TOT_STUD"
# Add fields to their corresponding FieldMap objects
fm_apt_poly_APT_ID.addInputField(outputfeatureAPT, "APT_ID")
fm_stud_ELEM.addInputField(outputfeatureSTUD, "ELEM")
fm_stud_INT_.addInputField(outputfeatureSTUD, "INT_")
fm_stud_MID.addInputField(outputfeatureSTUD, "MID")
fm_stud_HIGH.addInputField(outputfeatureSTUD, "HIGH")
fm_stud_TOT_STUD.addInputField(outputfeatureSTUD, "TOT_STUD")
# Set the output field properties for both FieldMap objects
apt_poly_APT_ID = fm_apt_poly_APT_ID.outputField
apt_poly_APT_ID.name = "APT_ID"
fm_apt_poly_APT_ID.outputField = apt_poly_APT_ID
stud_ELEM = fm_stud_ELEM.outputField
stud_ELEM.name = "ELEM"
stud_ELEM.aliasName = "ELEM"
fm_stud_ELEM.outputField = stud_ELEM
stud_INT_ = fm_stud_ELEM.outputField
stud_INT_.name = "INT_"
stud_INT_.aliasName = "INT_"
fm_stud_INT_.outputField = stud_INT_
stud_MID = fm_stud_MID.outputField
stud_MID.name = "MID"
stud_MID.aliasName = "MID"
fm_stud_MID.outputField = stud_MID
stud_HIGH = fm_stud_HIGH.outputField
stud_HIGH.name = "HIGH"
stud_HIGH.aliasName = "HIGH"
fm_stud_HIGH.outputField = stud_HIGH
stud_TOT_STUD = fm_stud_TOT_STUD.outputField
stud_TOT_STUD.name = "TOT_STUD"
stud_TOT_STUD.aliasName = "TOT_STUD"
fm_stud_TOT_STUD.outputField = stud_TOT_STUD
# Set the merge rule to sum and then replace the old field map in the mappings object with the updated one
fm_stud_ELEM.mergeRule = "SUM"
fm_stud_INT_.mergeRule = "SUM"
fm_stud_MID.mergeRule = "SUM"
fm_stud_HIGH.mergeRule = "SUM"
fm_stud_TOT_STUD.mergeRule = "SUM"
fms.addFieldMap(fm_apt_poly_APT_ID)
fms.addFieldMap(fm_stud_ELEM)
fms.addFieldMap(fm_stud_INT_)
fms.addFieldMap(fm_stud_MID)
fms.addFieldMap(fm_stud_HIGH)
fms.addFieldMap(fm_stud_TOT_STUD)
# Spatial Join Sample Code
# SpatialJoin_analysis (target_features, join_features, out_feature_class, {join_operation}, {join_type}, {field_mapping}, {match_option}, {search_radius}, {distance_field_name})
# Process: Spatial Join
arcpy.AddMessage("Performing Student Spatial Join")
arcpy.SpatialJoin_analysis(outputfeatureAPT, outputfeatureSTUD, output2, "JOIN_ONE_TO_ONE", "KEEP_ALL", fms, "INTERSECT", "", "")
arcpy.AddMessage("Just performed Student Spatial Join")
# Join the output to your apt_poly_xxxx by the APT_ID field
# JoinField_management Sample Code
# arcpy.JoinField_management(Input Feature, "Field Name", "join table", "join field", {["fields"]})
arcpy.AddJoin_management(target, "APT_ID", output2, "APT_ID","")
arcpy.AddMessage("Just performed Student Table Join")
# Calculate Field_management Sample Code
# arcpy.CalculateField_management(Input Feature, "Field Name", expression, "expression type", code block)
# Process: Calculate Field, with a string index in the expression used to isolate the feature class name
arcpy.CalculateField_management(target, targetname + ".ELEM_STUD", '!'+output2[74:]+'.ELEM!', "PYTHON_9.3")
# Process: Calculate Field (2)
arcpy.CalculateField_management(target, targetname + ".INT_STUD", '!'+output2[74:]+'.INT_!', "PYTHON_9.3")
# Process: Calculate Field (3)
arcpy.CalculateField_management(target, targetname + ".MID_STUD", '!'+output2[74:]+'.MID!', "PYTHON_9.3")
# Process: Calculate Field (4)
arcpy.CalculateField_management(target, targetname + ".HIGH_STUD", '!'+output2[74:]+'.HIGH!', "PYTHON_9.3")
# Process: Calculate Field (5)
arcpy.CalculateField_management(target, targetname + ".TOT_STUD", '!'+output2[74:]+'.TOT_STUD!', "PYTHON_9.3")
# Remove the Join
arcpy.RemoveJoin_management(target, "")
I eneded up deleting all fields in my target and join feature exept the onese that I needed, and then manually field mapped the rest. I was working with 6 fields and it was a pain, so I understand the idea of trying to work around doing it this way with 19 fields. But here's how it worked for me.