Count number of documents from MongoDB collection inside Array?

To count the number of documents from the collection inside the array, use aggregate(). Let us create a collection with documents −

> db.demo332.insertOne({details:[{Name:"Chris",Age:21}]});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e521f16f8647eb59e56208e")
}
> db.demo332.insertOne({details:[{Name:"David",Age:23},{Name:"Bob",Age:22}]});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e521f21f8647eb59e56208f")
}

Display all documents from a collection with the help of find() method −

> db.demo332.find();

This will produce the following output −

{ "_id" : ObjectId("5e521f16f8647eb59e56208e"), "details" : [ { "Name" : "Chris", "Age" : 21 } ] }
{ "_id" : ObjectId("5e521f21f8647eb59e56208f"), "details" : [ { "Name" : "David", "Age" : 23 }, { "Name" : "Bob", "Age" : 22 } ] }

Following is the query to count the number of documents from the collection inside an array −

> db.demo332.aggregate([
... {
...    $project: {
...       numberOfDocuments: { $cond: { if: { $isArray: "$details" }, then: { $size: "$details" }, else: "No Documents"} }
...    }
... }
... ] )

This will produce the following output −

{ "_id" : ObjectId("5e521f16f8647eb59e56208e"), "numberOfDocuments" : 1 }
{ "_id" : ObjectId("5e521f21f8647eb59e56208f"), "numberOfDocuments" : 2 }