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MongoDB query to display alternative documents with mapReduce() function and emit even field values
Let us create a collection with documents −
> db.demo636.insert({id:1});
WriteResult({ "nInserted" : 1 })
> db.demo636.insert({id:2});
WriteResult({ "nInserted" : 1 })
> db.demo636.insert({id:3});
WriteResult({ "nInserted" : 1 })
> db.demo636.insert({id:4});
WriteResult({ "nInserted" : 1 })
> db.demo636.insert({id:5});
WriteResult({ "nInserted" : 1 })
> db.demo636.insert({id:6});
WriteResult({ "nInserted" : 1 })
Display all documents from a collection with the help of find() method −
> db.demo636.find();
This will produce the following output −
{ "_id" : ObjectId("5e9c127b6c954c74be91e6d2"), "id" : 1 }
{ "_id" : ObjectId("5e9c127e6c954c74be91e6d3"), "id" : 2 }
{ "_id" : ObjectId("5e9c127f6c954c74be91e6d4"), "id" : 3 }
{ "_id" : ObjectId("5e9c12816c954c74be91e6d5"), "id" : 4 }
{ "_id" : ObjectId("5e9c12836c954c74be91e6d6"), "id" : 5 }
{ "_id" : ObjectId("5e9c12896c954c74be91e6d7"), "id" : 6 }
Following is the query to implement mapReduce() and display only the even values −
> db.demo636.mapReduce(
... function () {
... oddCounter++;
... var id= this._id;
... delete this._id;
... if ( oddCounter % d != 0 )
... emit(id, this );
... },
... function() {},
... {
... "scope": { "oddCounter": 0, "d": 2 },
... "out": { "inline": 1 }
... }
... )
This will produce the following output −
{
"results" : [
{
"_id" : ObjectId("5e9c127b6c954c74be91e6d2"),
"value" : {
"id" : 1
}
},
{
"_id" : ObjectId("5e9c127f6c954c74be91e6d4"),
"value" : {
"id" : 3
}
},
{
"_id" : ObjectId("5e9c12836c954c74be91e6d6"),
"value" : {
"id" : 5
}
}
],
"timeMillis" : 29,
"counts" : {
"input" : 6,
"emit" : 3,
"reduce" : 0,
"output" : 3
},
"ok" : 1
}
Updated on: 2020-05-12T07:54:12+05:30
150 Views
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