[Spring Data MongoDB] learning notes -- MapReduce, mongodb -- mapreduce

[Spring Data MongoDB] learning notes -- MapReduce, mongodb -- mapreduce

Mongodb MapReduce mainly includes two methods: map and reduce.

For example, assume that the following three records exist:

{ "_id" : ObjectId("4e5ff893c0277826074ec533"), "x" : [ "a", "b" ] }{ "_id" : ObjectId("4e5ff893c0277826074ec534"), "x" : [ "b", "c" ] }{ "_id" : ObjectId("4e5ff893c0277826074ec535"), "x" : [ "c", "d" ] }

The map method calls the emit method and returns the key-value pair. The key is the value of x [I], for example, a, and the value is 1.

function () {    for (var i = 0; i < this.x.length; i++) {        emit(this.x[i], 1);    }}

The reduce method collects statistics on key-value pairs.

function (key, values) {    var sum = 0;    for (var i = 0; i < values.length; i++)        sum += values[i];    return sum;}

The execution result is as follows:

{ "_id" : "a", "value" : 1 }{ "_id" : "b", "value" : 2 }{ "_id" : "c", "value" : 2 }{ "_id" : "d", "value" : 1 }

Assume that the map and reduce methods are stored in map. js and reduce. js respectively, and the mapreduce results can be obtained through the following code.

MapReduceResults<ValueObject> results = mongoOperations.mapReduce("jmr1", "classpath:map.js", "classpath:reduce.js", ValueObject.class);for (ValueObject valueObject : results) {  System.out.println(valueObject);}

The output is

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 code

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 + "]";  }}

You can add a MapReduceOptions to add some map-reduce options. The result is saved to the collection.

MapReduceResults<ValueObject> results = mongoOperations.mapReduce("jmr1", "classpath:map.js", "classpath:reduce.js",                                                                      new MapReduceOptions().outputCollection("jmr1_out"), ValueObject.class);

MapReduceOptions can also be instantiated using the static method options.

MapReduceResults<ValueObject> results = mongoOperations.mapReduce("jmr1", "classpath:map.js", "classpath:reduce.js",                                                                      options().outputCollection("jmr1_out"), ValueObject.class);

You can also add a query to filter statistics. Below we exclude documents containing a and B.

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);

 

An error occurred while inserting an array object in the spring data mongodb collection.

It seems like it overflows.

Teach Spring Data Mongodb auto-incremental ID implementation

You have to find a place to find the current number of IDs, and then query each insert operation, and add after insert.