I am starting to write a large web application using Clojure and
Compojure and am running into some design trouble while designing my
data model. To illustrate my problem I am going to make up some fake
data. Suppose you are writing an Insurance application which has the
tables Policy, Person and Vehicle. A policy has a person that is the
policy holder. A policy also has many vehicles. Each vehicle has a
person that is the primary driver. Most of the time the primary driver
is the same as the policy holder. If I were using one of the usual
object-relational mapping frameworks, (Hibernate, ActiveRecord) when I
load a policy I would get an object graph. If the person who is the
policy holder is the same as the person who is the primary driver of
the vehicle then the loaded Person object would be the same object. If
I change the address of the policy holder the primary driver's address
will also be changed.
How do people deal with this sort of thing in a Clojure application
(or any other functional language)? At first I thought that it would
be easy and I would just use nested maps. But this causes all kinds of
problems. If I load the data into nested maps I now have two distinct
maps for the same person. If I change one of them, the other is not
updated. If I try to save this map back to the database, which person
map has the correct data? It is also awkward to update the person in
the first place. In Java you would just go policy.getPolicyHolder
().setAddress("..."). In Clojure you would have to do something like
(assoc policy :holder (assoc (:holder policy) :address "...")).
I have a feeling that there is a more "functional" way to do this sort
of thing. My question is, how to other people deal with this? The only
thing that I can think of is that I would avoid using nested maps to
model the database associations. I would load the policy into a map.
Then if I need the person, I would load that into a separate map. That
may be the correct functional approach. I was just asking in case
there is some really cool thing that people do that I don't know
about.
I had a look at clj-record to see how associations where handled. It
looks like nested maps are avoided here. Instead functions are created
to retrieve the associated data. Is the correct way of this this?
Thank you,
Brenton
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