How to Create a nested index in MongoDB?

MongoDB automatically creates a multikey index if any indexed field is an array; you do not need to explicitly specify the multikey type.

This will work for both the scenario's

db.coll.createIndex( { "addr.pin": 1 } )

Scenario 1 nested OBJECTS

{
  userid: "1234",
  addr: {
    pin:"455522"
  }
},
{
  userid: "1234",
  addr: {
    pin:"777777"
  }
}

Scenario 2 nested Arrays

{
  userid: "1234",
  addr: [
    { pin:"455522" },
    { pin:"777777" },
  ]
}

https://docs.mongodb.com/manual/core/index-multikey/

You'd create them just as if you were creating an index on a top level field:

db.collection.createIndex({"attrs.nested.value": 1})

You do need to explicitly create indexes on each field.

A. to index all the properties in "nested" you will have to index them separately:

db.collection.createIndex({"attrs.nested.value": 1});
db.collection.createIndex({"attrs.nested.valuetwo": 1});

This can be done in one command with:

db.collection.createIndexes([{"attrs.nested.value": 1}, {"attrs.nested.valuetwo": 1}]);

B. to index just "valuetwo":

db.collection.createIndex({"attrs.nested.valuetwo": 1})

Use createIndex over ensureIndex as ensureIndex is Deprecated since version 3.0.0

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