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POHow to add a child object to a parent with App Engine?
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  1. POHow to add a child object to a parent with App Engine?
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    copied!<p>I have two classes in an owned one-to-many relationship. The parent is Map, and the child is POI (point of interest). I am trying to add a POI to an existing Map, but I get an exception when I try to persist my changes. Here are the two classes:</p> <pre><code>@PersistenceCapable(identityType = IdentityType.APPLICATION) public class Map { @PrimaryKey @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY) private Long id; @Persistent(mappedBy = "map") private List&lt;Poi&gt; pois; public List&lt;Poi&gt; getPois() { return pois; } } @PersistenceCapable(identityType = IdentityType.APPLICATION) public class Poi { @PrimaryKey @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY) private Key id; @Persistent private Map map; public Map getMap() { return map; } public void setMap(Map map) { this.map = map; } } </code></pre> <p>And here is how I am trying to use them:</p> <pre><code>PersistenceManager pm = PMF.get().getPersistenceManager(); // create a new POI Poi poi = new Poi(); // find the Map by its ID Map map = pm.getObjectById(Map.class, Long.decode(mapId)); // add the POI to the map map.getPois().add(poi); // persist! pm.makePersistent(map); pm.close(); </code></pre> <p>The line "map.getPois().add(poi);" throws an exception saying "java.lang.ClassCastException: java.lang.Long" but doesn't tell me why. If I switch it around to "poi.SetMap(map);" it just fails silently. There is no error message and nothing happens.</p> <p>Does anybody know how to correctly handle a one-to-many relationship in App Engine? Does anybody know of any good resources? <a href="http://code.google.com/appengine/docs/java/datastore/relationships.html" rel="nofollow noreferrer">Google's documentation</a> has been mildly helpful but it is really lacking.</p>
 

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