CODEWITHSUNDEEP
mongodbspring-mvcaggregation-frameworkmongotemplate
Deepak Pareek
Deepak Pareek

Reputation: 149

how to calculate average of a particular field in mongodb spring?

how to calculate average of a field in mongoDB and spring. we have $avg() function for terminal use but how to execute it with mongotemplate.

for example in 

 db.sales.aggregate(
    [
     {
       $group:
         {
           _id: "$item",
           avgAmount: { $avg: { $multiply: [ "$price", "$quantity" ] } },
           avgQuantity: { $avg: "$quantity" }
         }
     }
   ]
)

we are calculating average here so how can we execute it with mongotemplate.

Now I am using a function to get average rating

i am using function like this.. 

public List getrating() {

    TypedAggregation<RatingReviewModel> agg = newAggregation(RatingReviewModel.class,

           group("hospitalid")            
            .avg("rating").as("avgrating")
    );

    AggregationResults<DBObject> result = operations.aggregate(agg, DBObject.class);
    List<DBObject> resultList = result.getMappedResults();

return resultList;
}

but at the time of debugging resultList is Empty so it is returning nothing.

Upvotes: 2

Views: 4811

Answers (1)

chridam
chridam

Reputation: 103365

Suppose your Sale object is defined as:

class Sale {
    String id;
    String item;
    double price;
    int quantity;
}

Using the mongotemplate you would need a $project stage in the pipeline before hand to get the calculated fields, which can be a bit counter-intuitive because with the native MongoDB aggregation all is done in one $group operation pipeline rather than splitting the aggregation into two stages, thus:

import static org.springframework.data.mongodb.core.aggregation.Aggregation.*;

TypedAggregation<Sale> agg = newAggregation(Sale.class,
      project("quantity")
         .andExpression("price * quantity").as("totalAmount"),
      group("item")            
        .avg("totalAmount").as("avgAmount")
        .avg("quantity").as("avgQuantity")
);

AggregationResults<DBObject> result = mongoTemplate.aggregate(agg, DBObject.class);
List<DBObject> resultList = result.getMappedResults();

The above can also be achieved using the native Java Driver implementation:

ApplicationContext context = new AnnotationConfigApplicationContext(SpringMongoConfig.class);
MongoOperations operation = (MongoOperations) context.getBean("mongoTemplate");

BasicDBList pipeline = new BasicDBList();
String[] multiplier = { "$price", "$quantity" };

pipeline.add(
    new BasicDBObject("$group",
        new BasicDBObject("_id", "$item")
        .append("avgAmount", new BasicDBObject(
            "$avg", new BasicDBObject(
                "$multiply", multiplier
            )
        ))
        .append("avgQuantity", new BasicDBObject("$avg", "$quantity"))
    )
);

BasicDBObject aggregation = new BasicDBObject("aggregate", "sales")
                            .append("pipeline", pipeline);
System.out.println(aggregation);
CommandResult commandResult = operation.executeCommand(aggregation);

Upvotes: 2

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