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 }