Finder methods, named queries and relations
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Key: EJB-35
URL: http://jira.andromda.org/browse/EJB-35
Project: EJB Cartridge
Type: Improvement
Reporter: Sverker Abrahamsson
Assigned to: Vance Karimi
Priority: Trivial
Attachments: andromda-all.patch
Most of my queries involve related classes. For example, I have an entity
Subscription with a 0-n relation to entity User and a n-1 relation to entity
Channel. I want a finder method which returns all channels a given user is
subscribed to, without having to load neither the User nor the Subscription
entities (this is a quite trivial example, there are more complicated cases
where the difference acctually affects performance).
So I create a finder method in Channel called findSubscribedChannels(String
msisdn) where msisdn is primary key of User entity. This finder method should
return a collection of channels the given user is subscribed to.
The generated query is of course not correct but I thought it sholdn't be a
problem as I could override it in the DaoImpl class, but the problem is that
since it's created as a named query then JBoss verifies it when the application
is deployed and fails since Channel doesn't have any field named msisdn.
As the doc indicated that there are no possibility to override the generated
query except by creating a OCL query, I started to ponder how to solve this
issue (I later found that @andromda.ejb.query tagged value does work countrary
to what the doc says). I thought that if I instead specified the signature as
findSubscribedChannels(String subscriptions.user.msisdn) (where subscriptions
and user are the respective relation names) then the query generator could
detect that this finder argument refers to a field in a related entity. It
acctually worked out very conveniently. For this finder method a query like
this is generated:
select channel from Channel as channel inner join channel.subscriptions as
subscription where
subscription.user.msisdn = :subscriptionsUserMsisdn
Hence as long as just a simple query is neccesary then nothing else than
specifying the finder parameters results in a working query even when the query
involves several related enties.
To make it work I had to do a patch also for the main andromda project as the
signature of a finder method is created there but unless a operation parameter
name is allowed to have dot's in some context (I can't think of any) it
shouldn't affect anything negatively.
Now that I found out that I can override the named query with
@andromda.ejb.query maybe this patch isn't that neccesary but I still find it
convenient as it makes it visible directly in the model which attributes are
involved in the search.
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