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Retrieve the position in an Array in MongoDB?
You can use the concept of map reduce to get the position in an array. Let us first create a collection with documents −
> db.retrievePositionDemo.find();
{ "_id" : ObjectId("5cd569ec7924bb85b3f4893f"), "Subjects" : [ "MySQL", "MongoDB", "Java" ] }Following is the query to display all documents from a collection with the help of find() method −
> db.retrievePositionDemo.find().pretty();
This will produce the following output −
{
"_id" : ObjectId("5cd569ec7924bb85b3f4893f"),
"Subjects" : [
"MySQL",
"MongoDB",
"Java"
]
}Following is the query to retrieve the position in an array in MongoDB −
> db.retrievePositionDemo.mapReduce(
... function() {
... emit(this._id,{ "IndexValue": this.Subjects.indexOf("MongoDB") });
... },
... function() {},
... {
... "out": { "inline": 1 },
... "query": { "Subjects":"MongoDB"}
... }
... );This will produce the following output −
{
"results" : [
{
"_id" : ObjectId("5cd569ec7924bb85b3f4893f"),
"value" : {
"IndexValue" : 1
}
}
],
"timeMillis" : 662,
"counts" : {
"input" : 1,
"emit" : 1,
"reduce" : 0,
"output" : 1
},
"ok" : 1
}Look at the above sample output, the index or position of value “MongoDB” is 1. You can check for value “Java”, which is at position 2 −
> db.retrievePositionDemo.mapReduce(
... function() {
... emit(this._id,{ "IndexValue": this.Subjects.indexOf("Java") });
... },
... function() {},
... {
... "out": { "inline": 1 },
... "query": { "Subjects":"MongoDB"}
... }
... );This will produce the following output −
{
"results" : [
{
"_id" : ObjectId("5cd569ec7924bb85b3f4893f"),
"value" : {
"IndexValue" : 2
}
}
],
"timeMillis" : 30,
"counts" : {
"input" : 1,
"emit" : 1,
"reduce" : 0,
"output" : 1
},
"ok" : 1
}
Updated on: 30-Jul-2019
249 Views
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