Java 实现MongoDB Group 操作

在Java For  MongoDB 中时通过 DBCollectino对象的group 方法来实现Group 操作。

官方IPA:http://api.mongodb.org/java/2.7.3/

方法解释：

group

public DBObject group(DBObject key,
                      DBObject cond,
                      DBObject initial,
                      String reduce,
                      String finalize)
               throws MongoException

Applies a group operation

Parameters:

key - - { a : true }

cond - - optional condition on query

reduce - javascript reduce function

initial - initial value for first match on a key

finalize - An optional function that can operate on the result(s) of the reduce function.

Returns:

Throws:

MongoException

See Also:

http://www.mongodb.org/display/DOCS/Aggregation

案例实现：

final List<String> 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);