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POMorphia - issue with @Reference when using custom object for ID
 

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  1. POMorphia - issue with @Reference when using custom object for ID
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    <p>I am trying to use a custom object for entities' IDs instead of the usual ObjectId - the idea is to have a composite _id which would include a "version" along with the identifier.</p> <p>Here is what I would like to use for the ID:</p> <pre><code>public class VersionedId implements DBObject { ObjectId _id; Long _version; public Map toMap() { Map&lt;String, Object&gt; map = new HashMap&lt;String, Object&gt;(2); map.put("_id", _id); map.put("_version", _version ); return map; } } </code></pre> <p>The entities then look like this - parent referencing a sub-entity:</p> <pre><code>@Entity (value = "entities") public class VersionedEntity { @Id VersionedId _id; @Reference VersionedSubEntity subEntity; } </code></pre> <p>...and the sub-entity referenced:</p> <pre><code>@Entity (value = "subentities") public class VersionedSubEntity { @Id VersionedId _id; } </code></pre> <p>Both saving and loading of the VersionedSubEntity works fine, so does <em>saving</em> the VersionedEntity - the DBRef is correctly created with both the <code>_id</code> and <code>_version</code>. However, trying to <em>load</em> VersionedEntity throws the following:</p> <pre><code>java.lang.ClassCastException: com.mongodb.BasicDBObject cannot be cast to com.mongodb.DBRef at com.google.code.morphia.mapping.Mapper.fromDb(Mapper.java:483) at com.google.code.morphia.mapping.Mapper.fromDBObject(Mapper.java:267) at com.google.code.morphia.query.MorphiaIterator.processItem(MorphiaIterator.java:53) at com.google.code.morphia.query.MorphiaIterator.next(MorphiaIterator.java:48) at com.google.code.morphia.query.QueryImpl.get(QueryImpl.java:344) </code></pre> <p>Any hints on what I am doing wrong?</p>
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    1. PTQuestion
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    1. USPavel H
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    1. COwhy are you trying to do that? and have you taken a look at the @version annotation (used for optimistic locking, but you can simply reuse it for your own purpose)
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      1. POMorphia - issue with @Reference when using custom object for ID
      1. USxeraa
    2. CO@xeraa - I need to keep multiple versions of each entity/subentity in the DB, the entities then must know both the ID and the version of the referenced subentities for them to be uniquely identified, and more often than not they will refer to a _list_ of subentities, rather than a single one, so having the version as part of the ID/reference would be most convenient for me. (I do know about version annotation - doesn't help in this scenario I am afraid)
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      1. POMorphia - issue with @Reference when using custom object for ID
      1. USPavel H
    3. COSo this sounds like you're trying to map out a version control system in a database. Two thoughts on refactoring the design, using `@Reference(lazy=true)` which will prevent trying to load a chain of objects from a single parent, and ditching VersionId completely and just creating a new DBObject everytime you want to update it - technically creating a new 'version' of it while keeping the old one.
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      1. POMorphia - issue with @Reference when using custom object for ID
      1. USKappaMax
 

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