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POEquivalent of a repeated NDB StructuredProperty in JPA/JDO on Google App Engine
 

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  1. POEquivalent of a repeated NDB StructuredProperty in JPA/JDO on Google App Engine
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    <p>A list of <code>StructuredProperties</code> in NDB is modelled by a number of repeated properties, one repeated property per property of the <code>StructuredProperty's</code> model class. (See here: <a href="https://developers.google.com/appengine/docs/python/ndb/properties#structured." rel="nofollow">https://developers.google.com/appengine/docs/python/ndb/properties#structured.</a>)</p> <p>The closed equivalent I have found with JPA on Google App Engine is an <code>@Embedded</code> list of <code>@Embeddables</code>. The storage format, however, is now different. The GAE/J plugin for JPA generates one column per embedded entity per property of the embedded entity (see here: <a href="http://datanucleus-appengine.googlecode.com/svn-history/r979/trunk/src/com/google/appengine/datanucleus/StoreFieldManager.java" rel="nofollow">http://datanucleus-appengine.googlecode.com/svn-history/r979/trunk/src/com/google/appengine/datanucleus/StoreFieldManager.java</a>). (For long lists, this generates rows with many, many columns, for example.)</p> <p>Is there an easy built-in way to copy NDB's way to cope with repeated composite values using JPA or JDO?</p> <p>ADDED:</p> <p>For those more comfortable with Java than Python, I have found that the Java-based Objectify framework stores collections of embedded classes in the same way as NDB on Python: <a href="http://code.google.com/p/objectify-appengine/wiki/Entities#Embedding" rel="nofollow">http://code.google.com/p/objectify-appengine/wiki/Entities#Embedding</a>, which is the way I want to achieve with JPA (or JDO):</p> <p>For convenience, I am copying their example here:</p> <pre><code>import com.googlecode.objectify.annotation.Embed; import com.googlecode.objectify.annotation.Entity; import com.googlecode.objectify.annotation.Id; @Embed class LevelTwo { String bar; } @Embed class LevelOne { String foo; LevelTwo two } @Entity class EntityWithEmbeddedCollection { @Id Long id; List&lt;LevelOne&gt; ones = new ArrayList&lt;LevelOne&gt;(); } </code></pre> <p>Using this setup, run the following code:</p> <pre><code>EntityWithEmbeddedCollection ent = new EntityWithEmbeddedCollection(); for (int i=1; i&lt;=4; i++) { LevelOne one = new LevelOne(); one.foo = "foo" + i; one.two = new LevelTwo(); one.two.bar = "bar" + i; ent.ones.add(one); } ofy().save().entity(ent); </code></pre> <p>The resulting row layout (using repeated properties on the Datastore level) is then:</p> <p>ones.foo: ["foo1", "foo2", "foo3", "foo4"]</p> <p>ones.two.bar: ["bar1", "bar2", "bar3", "bar4"]</p>
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    1. USMarc
    1. USMarc
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    1. COdefine the java class(es) rather than referring to some Python syntax. When that is done then someone can respond on how to persist it using JDO or JPA
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    2. COAdded some code how it could be done with Objectify (which needs no mention of Python).
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      1. USMarc
 

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