Working With Futon : Creating Reduce Function

• In Couch database a view is a combination of  map and reduce function.
• In my previous post(link) we have seen how to create a custom map function.In this post we will see about reduce function.
• reduce function is used on each unique key of the result set of map function .It summarizes the result of map function and produces the specific result.
• In this demo, “a reduce function is created on student database.It collects all the fees of each student and returns the sum of fees .”
• a student object  properties in futon,

• A map function getAllStudent returns the properties of student objects with keys,

function(document) {

      
            atudent = {
                       name : document.name,
   
         marks : document.marks,
          
                       roll : document.roll,
 
                       course : document.course,
   
         fees : document.fees
                    };


            emit(document.roll, atudent );     
  
}

• The corresponding reduce function which sums up all the fees and returns it,

function(key, values, rereduce) {


var totalMoney = 0,

    studentList = new Array();

    for(i=0; i < values.length; i++) {
 
 totalMoney += values[i].fees;
    }
 
     return ("sum",totalMoney );

}

• The Futon view of (map,reduce) combination is, 

• Map Function result futon view, 

• Map Function result JSON view  ,

{ "offset" : 0,
  "rows" : [ { "id" : "326e49164d9b414a6c1ce2c8a802d28e",
        "key" : 3,
        "value" : { "course" : "BTECH",
            "fees" : 345,
            "marks" : 80,
            "name" : "sumanta",
            "roll" : 3
          }
      },
      { "id" : "326e49164d9b414a6c1ce2c8a802d175",
        "key" : 4,
        "value" : { "course" : "BTECH",
            "fees" : 345,
            "marks" : 75,
            "name" : "sandeep",
            "roll" : 4
          }
      },
      { "id" : "1a508cb01eab7f1ea315551a9703c20b",
        "key" : 5,
        "value" : { "course" : "MSC",
            "fees" : 467,
            "marks" : 267,
            "name" : "bapi",
            "roll" : 5
          }
      },
      { "id" : "326e49164d9b414a6c1ce2c8a802dda1",
        "key" : 7,
        "value" : { "course" : "SSC",
            "fees" : 123,
            "marks" : 95,
            "name" : "sangeeta",
            "roll" : 7
          }
      },
      { "id" : "326e49164d9b414a6c1ce2c8a802f200",
        "key" : 7,
        "value" : { "course" : "MSC IT",
            "fees" : 321,
            "marks" : 73,
            "name" : "Surabhi",
            "roll" : 7
          }
      },
      { "id" : "1a508cb01eab7f1ea315551a9703c5cf",
        "key" : 45,
        "value" : { "course" : "BTECH",
            "fees" : 567,
            "marks" : 345,
            "name" : "Dilip",
            "roll" : 45
          }
      }
    ],
  "total_rows" : 6
}

• Reduce Function result futon view(reduce check box selected),

•  Reduce Function result as JSON object,

{"rows":[
{"key":null,"value":2168}
]}