[Spring Data MongoDB]学习笔记－－MapReduce

mongodb的MapReduce主要包含两个方法：map和reduce。

举个例子，假设现在有下面3条记录

{ "_id" : ObjectId("4e5ff893c0277826074ec533"), "x" : [ "a", "b" ] }

{ "_id" : ObjectId("4e5ff893c0277826074ec534"), "x" : [ "b", "c" ] }

{ "_id" : ObjectId("4e5ff893c0277826074ec535"), "x" : [ "c", "d" ] }

map方法调用emit方法，返回键值对，键是x[i]的值，比如a; 值是1。

function () {

    for (var i = 0; i < this.x.length; i++) {

        emit(this.x[i], 1);

    }

}

reduce方法负责把键值对进行统计。

function (key, values) {

    var sum = 0;

    for (var i = 0; i < values.length; i++)

        sum += values[i];

    return sum;

}

执行结果如下

{ "_id" : "a", "value" : 1 }

{ "_id" : "b", "value" : 2 }

{ "_id" : "c", "value" : 2 }

{ "_id" : "d", "value" : 1 }

假设map和reduce方法分别存放在map.js和reduce.js中，可以通过下面的代码获得mapreduce结果。

MapReduceResults<ValueObject> results = mongoOperations.mapReduce("jmr1", "classpath:map.js", "classpath:reduce.js", ValueObject.class);

for (ValueObject valueObject : results) {

  System.out.println(valueObject);

}

输出是

ValueObject [id=a, value=1.0]

ValueObject [id=b, value=2.0]

ValueObject [id=c, value=2.0]

ValueObject [id=d, value=1.0]

ValueObject代码

public class ValueObject {



  private String id;

  private float value;



  public String getId() {

    return id;

  }



  public float getValue() {

    return value;

  }



  public void setValue(float value) {

    this.value = value;

  }



  @Override

  public String toString() {

    return "ValueObject [id=" + id + ", value=" + value + "]";

  }

}

可以增加一个MapReduceOptions来增加一些map－reduce的options，下面把结果存入collection。

MapReduceResults<ValueObject> results = mongoOperations.mapReduce("jmr1", "classpath:map.js", "classpath:reduce.js", 

                                                                     new MapReduceOptions().outputCollection("jmr1_out"), ValueObject.class);

MapReduceOptions也可以通过静态方法options()来实例化。

MapReduceResults<ValueObject> results = mongoOperations.mapReduce("jmr1", "classpath:map.js", "classpath:reduce.js", 

                                                                     options().outputCollection("jmr1_out"), ValueObject.class);

还可以加入一个query来对统计进行过滤，下面是把包含a和b的document排除在外。

Query query = new Query(where("x").ne(new String[] { "a", "b" }));

MapReduceResults<ValueObject> results = mongoOperations.mapReduce(query, "jmr1", "classpath:map.js", "classpath:reduce.js", 

                                                                     options().outputCollection("jmr1_out"), ValueObject.class);