Java 实现MongoDB Group 操作
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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 queryreduce
- javascript reduce functioninitial
- initial value for first match on a keyfinalize
- 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);
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