[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);