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$bsonSize (aggregation)

Definition

$bsonSize

New in version 4.4.

Returns the size in bytes of a given document (i.e. bsontype Object) when encoded as BSON. You can use $bsonSize as an alternative to the Object.bsonSize() method.

$bsonSize has the following syntax:

{ $bsonSize: <object> }

The argument can be any valid expression as long as it resolves to either an object or null. For more information on expressions, see Expressions.

Behavior

If the argument is an object, the expression returns the size of the object in bytes when the object is encoded as BSON.

If the argument is null, the expression returns null.

If the argument resolves to a data type other than an object or null, $bsonSize errors.

Examples

Return Sizes of Documents

From the mongo shell, create a sample collection named employees with the following documents:

 db.employees.insertMany([
   {
     "_id": 1,
     "name": "Alice", "email": "alice@company.com", "position": "Software Developer",
     "current_task": {
       "project_id": 1,
       "project_name": "Aggregation Improvements",
       "project_duration": 5,
       "hours": 20
     }
   },
   {
     "_id": 2,
     "name": "Bob", "email": "bob@company.com", "position": "Sales",
     "current_task": {
       "project_id": 2,
       "project_name": "Write Blog Posts",
       "project_duration": 2,
       "hours": 10,
       "notes": "Progress is slow. Waiting for feedback."
     }
   },
   {
     "_id": 3,
     "name": "Charlie", "email": "charlie@company.com", "position": "HR (On Leave)",
     "current_task": null
   },
   {
     "_id": 4,
     "name": "Dianne", "email": "diane@company.com", "position": "Web Designer",
     "current_task": {
       "project_id": 3,
       "project_name": "Update Home Page",
       "notes": "Need to scope this project."
     }
   }
]);

The following aggregation projects:

  • The name field
  • The object_size field, which uses $bsonSize to return the size of the document in bytes. The $$ROOT variable references the document currently being processed by the pipeline. To learn more about variables in the aggregation pipeline, see Variables in Aggregation Expressions.
db.employees.aggregate([
  {
    "$project": {
      "name": 1,
      "object_size": { $bsonSize: "$$ROOT" }
    }
  }
])

The operation returns the following result:

{ "_id" : 1, "name" : "Alice", "object_size" : 222 }
{ "_id" : 2, "name" : "Bob", "object_size" : 248 }
{ "_id" : 3, "name" : "Charlie", "object_size" : 112 }
{ "_id" : 4, "name" : "Dianne", "object_size" : 207 }

Return Combined Size of All Documents in a Collection

The following pipeline returns the combined size of all of the documents in the employees collection:

db.employees.aggregate([
  {
    "$group": {
      "_id": null,
      "combined_object_size": { $sum: { $bsonSize: "$$ROOT" } }
    }
  }
])

When you specify an $group _id value of null, or any other constant value, the $group stage calculates accumulated values for all the input documents as a whole.

The operation uses the $sum operator to calculate the combined $bsonSize of each document in the collection. The $$ROOT variable references the document currently being processed by the pipeline. To learn more about variables in the aggregation pipeline, see Variables in Aggregation Expressions.

The operation returns the following result:

{ "_id" : null, "combined_object_size" : 789 }

See also

Return Document with Largest Specified Field

The following pipeline returns the document with the largest current_task field in bytes:

db.employees.aggregate([
   // First Stage
   { $project: { name: "$name", task_object_size: { $bsonSize: "$current_task" } }  },
   // Second Stage
   { $sort: { "task_object_size" : -1 } },
   // Third Stage
   { $limit: 1 }
])
First Stage

The first stage of the pipeline projects:

  • The name field
  • The task_object_size field, which uses $bsonSize to return the size of the document’s current_task field in bytes.

This stage outputs the following documents to the next stage:

{ "_id" : 1, "name" : "Alice", "task_object_size" : 109 }
{ "_id" : 2, "name" : "Bob", "task_object_size" : 152 }
{ "_id" : 3, "name" : "Charlie", "task_object_size" : null }
{ "_id" : 4, "name" : "Dianne", "task_object_size" : 99 }
Second Stage

The second stage sorts the documents by task_object_size in descending order.

This stage outputs the following documents to the next stage:

{ "_id" : 2, "name" : "Bob", "task_object_size" : 152 }
{ "_id" : 1, "name" : "Alice", "task_object_size" : 109 }
{ "_id" : 4, "name" : "Dianne", "task_object_size" : 99 }
{ "_id" : 3, "name" : "Charlie", "task_object_size" : null }
Third Stage

The third stage limits the output documents to only return the document appearing first in the sort order:

{ "_id" : 2, "name" : "Bob", "task_object_size" : 152 }