[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的值，比如a; 值是1。
function () {
for (var i = 0; i < this.x.length; i++) {
emit(this.x, 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;
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 results = mongoOperations.mapReduce("jmr1", "classpath:map.js", "classpath:reduce.js", ValueObject.class);
for (ValueObject valueObject : results) {
System.out.println(valueObject);
}
输出是
ValueObject
ValueObject
ValueObject
ValueObject
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 ";
}
}
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 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 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 results = mongoOperations.mapReduce(query, "jmr1", "classpath:map.js", "classpath:reduce.js",
options().outputCollection("jmr1_out"), ValueObject.class);
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