On Nov 12, 2008, at 6:38 AM, John Pannell wrote:
subway stations all over town). Each station has two or more "port"
entities (i.e. the openings in and out of the station), which each
have one or more "pipe" entities (i.e. the tunnels that connect the
stations to one another). A single port might have more than one
pipe in the case that there is a fork in the tunnel, and a train
exiting the port could end up in one of a variety of stations.
The station has a to-many "ports" relationship with a cascade delete
rule, and the port has a to-one "station" relationship with a
cascade delete rule - these are inverses on one another. So now, if
I delete a station, all of its ports will also be deleted from the
object graph. Deleting a port, on the other hand, will remove it
from the ports relationship on the station, but leave the station
otherwise intact.
My trouble comes in relating the ports and the pipes. I'd like to
model the pipe by having a to-one "sourcePort" relationship (to
represent the port at the start of the tunnel - note: all of my
tunnels are one-way) and a to-one "targetPort" relationship to
represent the port at the end of the tunnel. A port would have a to-
many relationship with pipe as the target entity. The difficulty
comes in specifying an inverse... for the port's "pipes"
relationship, I'm forced to choose from either the sourcePort or the
targetPort as the inverse, and for the pipe's sourcePort or
targetPort relationships, setting either to port's "pipes" breaks
the inverse setting of the other.
You may be over-thinking Core Data's use of inverse relationships.
Core Data wants inverse relationships to maintain graph integrity,
such relationships that don't have to have anything to do with your
application logic. Although subtle, I find this a clarifying
distinction.
Anyway, I would recommend a multiple relationship setup:
1. Each pipe should have a to-one with port called "sourcePort", the
inverse would be a to-many called "sourcePipes"
2. Each pipe should have a to-many with port called "targetPorts", the
inverse would be a to-one called "targetPipe"
HTH,
Keary Suska
Esoteritech, Inc.
"Demystifying technology for your home or business"
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