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POHow do I pass parameters to Mongodb map/reduce in Java?
 

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  1. POHow do I pass parameters to Mongodb map/reduce in Java?
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    <p>I have some data like this:</p> <pre><code>{id: 1, text: "This is a sentence about dogs", indices: ["sentence", "dogs"]} {id: 2, text: "This sentence is about cats and dogs", indices: ["sentence", "cats", "dogs"]} </code></pre> <p>Where I have manually extracted key terms from the text and stored them as indices. I want to be able to do a search and order the results with the most matching indices. So for this example, I would like to be able to pass "cats" and "dogs" and get both objects returned, but id=2 should be first with score=2.</p> <p>I first tried to use the DBCollection.group function</p> <p>{<code>public DBObject group(DBObject key, DBObject cond, DBObject initial, String reduce, String finalize)</code>}</p> <p>But I don't see a way to send parameters. I tried:</p> <pre><code>key: {id: true}, cond: {"indices" $in ['cats', 'dogs']}, initial: {score: 0} reduce: function(doc, out){ out.score++; } </code></pre> <p>but obviously this will just return a count of 1 for each of the 2 objects.</p> <p>I realised that I could send the keyword parameters as part of the initial config of the reduced object.</p> <pre><code>final List&lt;String&gt; targetTerms = Arrays.asList("dogs", "cats"); final Datastore ds = …. final DBCollection coll = ds.getCollection(Example.class); BasicDBObject key = new BasicDBObject("_id", true); BasicDBObject cond = new BasicDBObject(); cond.append("indices", new BasicDBObject("$in", targetTerms)); BasicDBObject initial = new BasicDBObject(); initial.append("score", 0); initial.append("targetTerms", targetTerms); String reduce = "function (obj, prev) { " + " for (i in prev.targetTerms) {" + " targetTerm = prev.targetTerms[i];"+ " for (j in obj.indices) {" + " var index = obj.indices[j];"+ " if (targetTerm === index) prev.score++;" + " }" + " }" + "}"; String fn = null; final BasicDBList group = (BasicDBList) coll.group(key, cond, initial, reduce, fn); </code></pre> <p>I get results like this:</p> <pre><code>{ "_id" : { "$oid" : "4dcfe16c05a063bb07ccbb7b"} , "score" : 1.0 , "targetTerms" : [ "virtual" , "library"]} { "_id" : { "$oid" : "4dcfe17d05a063bb07ccbb83"} , "score" : 2.0 , "targetTerms" : [ "virtual" , "library"]} </code></pre> <p>This got me the score values that I wanted, and I am able to narrow down the entries to be processed with more specific conditional rules.</p> <p>So I have a few questions:</p> <ol> <li>Is this a good way to send "parameters" to the group action's reduce function?</li> <li>Is there a way to sort (and perhaps limit) the output inside mongodb before returning to the client?</li> <li>Will this break on sharded Mongodb instances?</li> </ol>
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    1. PTQuestion
    1. This table or related slice is empty.
    1. USNic Cottrell
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    1. CORe 2: By default the map/reduce result is sorted by the id, that is why you see the sort that you see. However when running Map/Reduce in mongodb, you can specify a target collection instead of returning results to the client. Then you should be able to query that collection and specify query and sort as with regular queries.
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      1. POHow do I pass parameters to Mongodb map/reduce in Java?
      1. USKen Egozi
    2. COThanks Ken. But how do I specify the target collection with the GroupCommand in Java? I don't see any mention at http://api.mongodb.org/java/current/com/mongodb/GroupCommand.html
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      1. POHow do I pass parameters to Mongodb map/reduce in Java?
      1. USNic Cottrell
    3. COGroup does not support "out". http://www.mongodb.org/display/DOCS/Aggregation#Aggregation-Group
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      1. POHow do I pass parameters to Mongodb map/reduce in Java?
      1. USGates VP
 

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