Hi all-
Looking for advice on the following scenario: consider an object graph
made up of entities called "stations" (i.e. like 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.
I can model this many-to-many between ports and pipes with some
cardinality rules for the pipe (only two ports for a pipe), but then
it feels like I'm doing extra work in code by having to determine
which of a pipe's ports is the sourcePort, and which is the
targetPort. Alternately, I could leave the relationships
unidirectional and be sure to delete attached pipes when deleting a
port, and remove attached pipes from port's pipes set when deleting a
pipe.
The core data documentation speaks strongly against unidirectional
relationships... can anyone suggest better modeling for this
scenario? Or testify to the soundness of unidirectional relationships
if managed as described? Any help is appreciated!
John
John Pannell
Positive Spin Media
http://www.positivespinmedia.com
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