[Spring Data MongoDB] Learning Note--mapreduce

MongoDB's mapreduce mainly consists of two methods: Map and reduce.

For example, let's say we have the following 3 records

{"_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, returns the value of the key value pair, the key is x[i], such as A; The value is 1.

function () {    for (var0this. x.length; i++) {        emit (   This 1 );    }}

The reduce method is responsible for the statistics of key-value pairs.

function (key, values) {    var0;      for (var0; i < values.length; i++)         + = Values[i]    ; return sum;}

Execution results are as follows

{"_id":"a","value":1 }{ "_id":"b","value":2 }{ "_id":"C","value":2 }{ "_id":"D","value":1}

Assuming that the map and reduce methods are stored in Map.js and Reduce.js, the MapReduce results can be obtained from 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);}

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 classValueobject {PrivateString ID; Private floatvalue;  PublicString getId () {returnID; }   Public floatGetValue () {returnvalue; }   Public voidSetValue (floatvalue) {     This. Value =value; } @Override PublicString toString () {return"Valueobject [id=" + ID + ", value=" + value + "]"; }}

You can add a mapreduceoptions to add some map-reduce options, and the results are stored in 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 by means of 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 the statistics, which excludes the document containing A and B.

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);