[castor-dev] 0.9.3.9 collection incompatibility

[Todd V. Jonker]

Hello,

I'm attempting to migrate from Castor 0.9.3 to 0.9.3.9 and have discovered
some strange problems.  I'm using Castor JDO on top of PostreSQL.

My problem is that a mapping file that worked fine now fails to work
properly with regards to the collections used for many-to-many mappings.

I get this error:

    [junit] org.exolab.castor.mapping.MappingException: The return type for
method public java.util.List inpath.mdpath.ScheduleTemplate.getShifts() does
not match the declared field type java.util.Collection

Here's the relevant mapping snippet:

--------
mapping.xml
--------
<class name="inpath.mdpath.ScheduleTemplate"
         identity="id"
         key-generator="MAX">
    <map-to table="schedule_template" />
    <field name="id" type="integer" >
      <sql name="id" type="integer"/>
    </field>

    <field name="shifts" type="inpath.mdpath.castor.CastorShift"
           required="true" collection="collection">
      <sql many-table="schedule_template_shift"
           many-key="template_id"
           name="shift_id" />
    </field>
    <field name="period" type="integer">
      <sql name="minutes_per_row" type="integer" />
    </field>
  </class>
--------

And the related code:

--------
ScheduleTemplate.java
--------
public class ScheduleTemplate
    extends Timeline
{
    public List getShifts()
    {
        ... Return an (unmodifiable) ArrayList ...
    }

    public void setShifts(List shifts)
    {
        ... Copy values from shifts into internal ArrayList ...
    }

    public void addShift(Shift shift)
    {
        ... Add shift to my internal ArrayList ...
    }

    public void removeShift(Shift shift)
    {
        ... You can guess ...
    }
}
--------

My first question is:  why does this fail where it worked fine before?  I
have quite a bit of code that uses the idiom of get/set via List.

In trying to find a solution, I saw the following snippet in the ChangeLog:

--------
XML & JDO:
    Now the return type of get-method must be a super-class of or equal to
    the class declared in mapping.xml (was: sub-class).
    Note, that now "arraylist" can be used as a value of "collection"
    attribute in mapping.xml
--------

Can someone in-the-know please explain what in blazes this means?  It sounds
like a direct violation of the covariance principle.

Seeing the note about arraylist, I modified the mapping accordingly (since
ArrayList is what I'm actually using anyway).  Now I get a different error:

--------
    [junit] org.exolab.castor.mapping.MappingException: The method
set/addShifts in class inpath.mdpath.ScheduleTemplate accepting/returning
object of type class inpath.mdpath.castor.CastorShift was not found
--------

Another WTF moment. I've seen this horrible, incomprehensible error message
before, and now it just makes no sense whatsoever.

I've tried to find documentation on how to do this, but as far as I can
tell, the 1-N and N-M mapping design is almost entirely neglected except for
the very simplistic example code.  For example, when is a single-element
addFoo(Foo) method used instead of the setFoos(List) ?  I've seen both
called at different times, and I can't figure out what the rules are.

If anybody can explain exactly how this is supposed to work, what the types
should be, in the various places, I'd greatly appreciate it.  I'm having a
show-stopping inexplicable problem with 0.9.3 and I'd like to migrate to the
newer code in hopes that it goes away!

Thanks,
.T.

Todd V. Jonker
Inpath Solutions, LLC
www.inpathsol.com

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