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POHow do I perform atomic updates of fields of a MongoDB document?
 

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  1. POHow do I perform atomic updates of fields of a MongoDB document?
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    <p>Disclaimer: I'm a noob to Mongo and to databases in general, so if I'm off on terminology or concepts please let me know.</p> <p>For the purposes of this example, I'd like to store the mouse coordinates together within an object nested within a document. That document would look something like:</p> <pre><code>{ "_id" : ObjectId("4f39805d35919a2a7c7aba3e"), "mouseLoc" : { "x" : 10, "y" : 20 } } </code></pre> <p>It seems like <a href="http://www.mongodb.org/display/DOCS/Updating#Updating-ModifierOperations" rel="nofollow">atomic updating, via modifier operations</a>, is the way to go, as I'm only storing values here (retrieving them elsewhere, in other software). However, I can't figure out the query part. How do I access this document in order to set the x and y values?</p> <p>I'm using Java, so I'm currently trying:</p> <pre><code>BasicDBObject mouseLoc = new BasicDBObject(); mouseLoc.put("x", mouseX); mouseLoc.put("y", mouseY); myCollection.update(queryObj, new BasicDBObject("$set", mouseLoc), true, false); </code></pre> <p>However, I don't know how to specify that queryObj to get the document with the mouseLoc key. Alternatively, if there's a smarter way to store information like this, please school me.</p> <p>I can follow along with Mongo shell/JS tips if that's simpler.</p> <p><br/> <strong>UPDATE:</strong><br/> I can query for this document if I give it a static field like 'name'. So, to update this document:</p> <pre><code>{ "_id" : ObjectId("4f39805d35919a2a7c7aba3e"), "name" : "mouseLoc", "mouseLoc" : { "x" : 10, "y" : 20 } } </code></pre> <p>I can use this code:</p> <pre><code>BasicDBObject mouseLoc = new BasicDBObject(); mouseLoc.put("x", mouseX); mouseLoc.put("y", mouseY); BasicDBObject queryObj = new BasicDBObject("name", "mouseLoc"); BasicDBObject updateObj = new BasicDBObject("$set", new BasicDBObject("mouseLoc", mouseLoc)); myCollection.update(queryObj, updateObj, true, false); </code></pre> <p>However, it seems redundant to have to specify that "name" field just to be able to retrieve the document with the "mouseLoc" key. Maybe I'm just misunderstanding database / mongo design?</p>
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