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How to get the index of an array element in older versions on MongoDB?
To get index of an array element, use $indexOfArray. Let us create a collection with documents −
> db.demo65.insertOne({"ListOfValues":[10,20,30]}); { "acknowledged" : true, "insertedId" : ObjectId("5e28990ecfb11e5c34d89938") } > db.demo65.insertOne({"ListOfValues":[50,60,70,100]}); { "acknowledged" : true, "insertedId" : ObjectId("5e28991ecfb11e5c34d89939") } > db.demo65.insertOne({"ListOfValues":[30,40,89,91,98]}); { "acknowledged" : true, "insertedId" : ObjectId("5e28992bcfb11e5c34d8993a") }
Display all documents from a collection with the help of find() method −
> db.demo65.find();
This will produce the following output −
{ "_id" : ObjectId("5e28990ecfb11e5c34d89938"), "ListOfValues" : [ 10, 20, 30 ] } { "_id" : ObjectId("5e28991ecfb11e5c34d89939"), "ListOfValues" : [ 50, 60, 70, 100 ] } { "_id" : ObjectId("5e28992bcfb11e5c34d8993a"), "ListOfValues" : [ 30, 40, 89, 91, 98 ] }
Here is the query to get the index of an array element in older versions on MongoDB −
> db.demo65.aggregate( ... [ ... { ... $project: ... { ... IndexOfAnArrayElement: { $indexOfArray: [ "$ListOfValues", 30] }, ... } ... } ... ] ... );
This will produce the following output −
{ "_id" : ObjectId("5e28990ecfb11e5c34d89938"), "IndexOfAnArrayElement" : 2 } { "_id" : ObjectId("5e28991ecfb11e5c34d89939"), "IndexOfAnArrayElement" : -1 } { "_id" : ObjectId("5e28992bcfb11e5c34d8993a"), "IndexOfAnArrayElement" : 0 }
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