How To Use Map Reduce in Spring Mongo data

Suppose  I have order table and I want to find out best five selling product.

Create a method that returns best product of ValueObject Type .

ValueObject Type basically contain id and corresponding values to those ids as shown below:

public class ValueObject implements Comparable<ValueObject>{

private Object id;

private int value;

public Object getId() {

return id;

}

public void setId(Object id) {

this.id = id;

}

public int getValue() {

return value;

}

public void setValue(int value) {

this.value = value;

}

@Override

public String toString() {

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

}

@Override

public int compareTo(ValueObject o) {

if(o instanceof ValueObject){

return o.getValue()-this.getValue();

}

else{

throw new ClassCastException();

}

}

}

Now, you can call mapReduce method on MongoTemplate which return result as MapReduceResult

public Iterable<ValueObject> countBestSellingProduct() {

MapReduceResults<ValueObject> results = mongoTemplate.mapReduce("order", "map.js", ”reduce.js", ValueObject.class);

             

              return results;

       }

Syntax of mapReduce ()

mapReduce(“Collection Name”, “ mapfunction”, “reducefunction”, ValueObect.class);

* You can also pass your query to mapReduce then Sytax will be

mapReduce(query, “Collection Name”,  “mapfunction”, “reducefunction”, ValueObect.class);

map.js (This is just an example to map with id)

function ()

{

       var prt = this.$id;

       if(prt != null ){

             

       }

       emit(this.$id, 1);

}

reduce.js(Example of a reduce function)

function (key, values)

{

       var sum = 0;

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

       {

              sum += values[i];

       }

       return sum;

}

You will get result which is Iterable ValueObject type

Now Convert it into List<ValueObject>

List<ValueObject> lst = new ArrayList<ValueObject>();

              for (ValueObject prt: result) {

                     lst.add(prt);

                     

              }

And finally, sort this list by using

Collections.sort(lst);

Now, you can top five or more best-selling products by taking out a sublist from the above list.

List<ValueObject> bestFiveSellingProduct=lst.subList(0,4)

Simple, isn' it??

Cheers !!!!!!!!!!!!!!!!!!!!!!!