Join attributes of 2 points to 1 line [duplicate]

Question

I have a set of lines that were created as lines between point pairs. Now I need to know, which points the lines connect. I thus want to join the Ids of both points to the line. So far I tried to use spatial join, but one ID is always right, while the other ID is always wrong. The lines were created from the points, so they mark the vertices of the lines.
In the end I want to have a single ID, derived from both IDs. For this question I only need to know how to get 2 columns in the line table with the IDs from the vertice points.

How can I (spatial) join two points to one line?

I am using ArcGIS 10.3 on Windows 7.

Answer 1

Use Spatial Join with some Python:

import arcpy
from collections import defaultdict
arcpy.env.overwriteOutput = 1

#Change these lines to match the names of the feature layers and the field names
points = 'Points'
pointidfield = 'PointIDField'
lines = 'Points_PointsToLine'
linefield1 = 'Point1'
linefield2 = 'Point2'

spatjoin = r'in_memory\spatjoin'
arcpy.SpatialJoin_analysis(target_features=lines, join_features=points, 
                          out_feature_class=spatjoin, join_operation='JOIN_ONE_TO_MANY')

d = defaultdict(list)

for key,value in arcpy.da.SearchCursor(spatjoin,['TARGET_FID',pointidfield]):
    d[key].append(value)

with arcpy.da.UpdateCursor(lines,['OID@',linefield1,linefield2]) as cursor:
    for row in cursor:
        row[1:] = d[row[0]]
        cursor.updateRow(row)