Re: Auto generation of the WHERE clause

[shoevarek <va...@gmail.com>]

I have played with IBM version strategy and acheived desired final outcome by
using @VersionStrategy and explicit @Version field and set such field to not
updatable/insertable (an exception being thrown when optimistic locking is
detected) however I realised I might be using optimistic locking not in the
way it was intended by JPA standard. 

JPA allows defining version fields and manages that field value through
different strategy implementations. It works fine, throwing optimistic
locking exception, when the data in the database has been changed since the
entity has been loaded into persistence context within the context of the
same transaction. Problem is that I have two applications - web application
and backend web service application. The former retrieves data by accessing
the latter, shows it on UI allowing user make changes and finally updates
database by again calling the web service. The user can work on the form for
a long time and I have to detect scenarios when the same data has been
modified by other processes. On the UI form (or cached in web app session) I
keep version information for my entities which I then pass back in update
web service request. My web service then finds entity by its id and sets
properties of that managed entity with new values (the form modifies only
subset of fields) including original version value (which is passed back).
When the value of the version field was modified in the database I am
getting an exception, but it is not optimistic locking exception but
exception caused by changing version field value, something that is not
allowed by JPA. I achieve desired outcome, which is an exception, but it is
not the exception type I want. 

Example:

Entity e = em.findById(id);
e.setAddress(new value);
e.setVersion(old value); // causes an exception due to value strategy
"restrict"

I tried alternative approach, where instead of finding the entity and
updating state of the managed entity I create new entity instance and then
merge it. In such case I can set version field as entity is still detached
and I get actual optimistic locking exception.Example:

Entity e = new Entity();
e.setId(id);
e.setAddress(new value);
e.setVersion(old value);
em.merge(e);

Is it then the only correct way of using JPA optimistic locking mechanism
(in my specific use case), meaning it is only appropriate to update database
state through merge entity operation?




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