Multiple Field Key to Single Field Key for Relates using Python and ArcPy

MultiField_to_SingleKey.py

#Name: Multiple Field Key to Single Field Key for Relates using Python and ArcPy
#Author: Bryan McIntosh
#Description: For updating a single LINK_ID field in both the parent and child table
#             based on a natural 4 column composite index relationship.
import arcpy
## VARIABLES ##
edit_ws = r'C:\SpatialTimes.gdb' #Edit workspace
xs_fc = r'C:\SpatialTimes.gdb\CrossSection' #parent FC
xs_tbl = r'C:\SpatialTimes.gdb\CrossSection_RatingCurves' #related table
xs_att = ['Link_ID','F1','F2','F3','F4','OID@']
## FUNCTIONS ##
def fnUpdateXSTBL(qry, linkID):
    with arcpy.da.UpdateCursor(xs_tbl,xs_att, qry) as cursor2:
            for row in cursor2:
                row[0] = linkID
                cursor2.updateRow(row)
## MAIN CODE ##
edit = arcpy.da.Editor(edit_ws) #Set the edit workspace
edit.startEditing(False, False) #Note: change second param to true if using SDE WS
with arcpy.da.UpdateCursor(xs_fc, xs_att) as cursor:
    for row in cursor:
        row[0] = row[5] #Makes the LinkID equal to the current ObjectID of parent
        cursor.updateRow(row) #Update the parent row
        #Set the query for the multi-field natural key (fields are F1, F2, F3, F4)
        xs_tbl_qry = xs_att[1] + " = '{0}' AND ".format(row[1]) + xs_att[2] + " = '{0}' AND ".format(row[2]) + xs_att[3] + " = '{0}' AND ".format(row[3]) + xs_att[4] + " = {0}".format(row[4])
        fnUpdateXSTBL(xs_tbl_qry, row[5]) # Send the query and parent ObjectID to update the related table
edit.stopEditing(True)

Hey @elbreidenbach, I haven't looked at this in a few years, but at first glance you could check if there are related records before using cursor2. Many approaches - select statement and count, query layer, or a searchCursor (easy since uses the same syntax).

I haven't test this but try adding a new first line inside the fnUpdateXSTBL function. Use the python any() function to evaluate if there is a selection using the same logic as the updateCursor (would return true if results). If so, cursor2 to the races.

if any(arcpy.da.SearchCursor(xs_tbl,xs_att, qry)):

Hi @TheBryanMac,

Thanks for your help! The any with a SearchCursor works for me. I appreciate your super quick response. On to the next challenge.