MongoDB indexes every value of an array so that you can query for single elements.
To understand the concept, let us create a collection with the document. The query to create a collection with a document is as follows −
> db.indexingForArrayElementDemo.insertOne({"StudentFavouriteSubject":["MongoDB","MySQL"]}); { "acknowledged" : true, "insertedId" : ObjectId("5c8acdca6cea1f28b7aa0816") }
Display all documents from a collection with the help of find() method. The query is as follows −
> db.indexingForArrayElementDemo.find().pretty();
The following is the output −
{ "_id" : ObjectId("5c8acdca6cea1f28b7aa0816"), "StudentFavouriteSubject" : [ "MongoDB", "MySQL" ] }
Here is the query by which MongoDB index array &minus ;
> db.indexingForArrayElementDemo.ensureIndex({"StudentFavouriteSubject":1});
The following is the output −
{ "createdCollectionAutomatically" : false, "numIndexesBefore" : 1, "numIndexesAfter" : 2, "ok" : 1 }
Here is the query for individual array elements −
> db.indexingForArrayElementDemo.find({"StudentFavouriteSubject":"MongoDB"}).pretty();
The following is the output −
{ "_id" : ObjectId("5c8acdca6cea1f28b7aa0816"), "StudentFavouriteSubject" : [ "MongoDB", "MySQL" ] }
Let us see another example. The query is as follows −
> db.indexingForArrayElementDemo.find({"StudentFavouriteSubject":"MySQL"}).pretty();
The following is the output −
{ "_id" : ObjectId("5c8acdca6cea1f28b7aa0816"), "StudentFavouriteSubject" : [ "MongoDB", "MySQL" ] }