Count Documents
Overview
In this guide, you can learn how to get an accurate and estimated count of the number of documents in your collection.
Sample Data
The examples in this guide use the following documents in a collection called
students
:
{ "_id": 1, "name": "Jonathon Howard ", "finalGrade": 87.5 } { "_id": 2, "name": "Keisha Freeman", "finalGrade": 12.3 } { "_id": 3, "name": "Wei Zhang", "finalGrade": 99.0 } { "_id": 4, "name": "Juan Gonzalez", "finalGrade": 85.5 } { "_id": 5, "name": "Erik Trout", "finalGrade": 72.3 } { "_id": 6, "name": "Demarcus Smith", "finalGrade": 88.8 }
The following Student
class models the documents in this
collection:
public class Student { public int Id { get; set; } public string Name { get; set; } public double FinalGrade { get; set; } }
Note
The documents in the students
collection use the camel-case naming
convention. The examples in this guide use a ConventionPack
to deserialize the fields in the collection into Pascal case and map them to
the properties in the Student
class.
To learn more about custom serialization, see Custom Serialization.
Accurate Count
To count the number of documents that match your query filter, use the
CountDocuments()
method. If you pass an empty query filter, this method
returns the total number of documents in the collection.
Example
The following example counts the number of documents where the
value of finalGrade
is less than 80
:
var filter = Builders<Student>.Filter.Lt(s => s.FinalGrade, 80.0); var count = _myColl.CountDocuments(filter); Console.WriteLine("Number of documents with a final grade less than 80: " + count);
Number of documents with a final grade less than 80: 2
Modify Behavior
You can modify the behavior of CountDocuments()
by passing a CountOptions
type as
a parameter. If you don't specify any options, the driver uses default values.
You can set the following properties in a CountOptions
object:
Property | Description |
---|---|
Collation | The type of language collation to use when sorting results. Default: null |
Hint | The index to use to scan for documents to count. Default: null |
Limit | The maximum number of documents to count. Default: 0 |
MaxTime | The maximum amount of time that the query can run on the server. Default: null |
Skip | The number of documents to skip before counting. Default: 0 |
Tip
When you use CountDocuments()
to return the total number of documents in a
collection, MongoDB performs a collection scan. You can avoid a collection scan and
improve the performance of this method by using a hint to take advantage of the built-in index on
the _id
field. Use this technique only when calling CountDocuments()
with an empty query parameter.
var filter = Builders<Student>.Filter.Empty; CountOptions opts = new CountOptions(){Hint = "_id_"}; var count = collection.CountDocuments(filter, opts);
Estimated Count
To estimate the total number of documents in your collection, use the
EstimatedDocumentCount()
method.
Note
The EstimatedDocumentCount()
method is more efficient than the
CountDocuments()
method because it uses the collection's
metadata rather than scanning the entire collection.
Modify Behavior
You can modify the behavior of EstimatedDocumentCount()
by passing a
EstimatedDocumentCountOptions
type as a parameter. If you don't
specify any options, the driver uses default values.
You can set the following properties in a EstimatedDocumentCountOptions
object:
Property | Description |
---|---|
MaxTime | The maximum amount of time that the query can run on the server. Default: null |
Example
The following example estimates the number of documents in the
students
collection:
var count = _myColl.EstimatedDocumentCount(); Console.WriteLine("Estimated number of documents in the students collection: " + count);
Estimated number of documents in the students collection: 6
Aggregation
You can use the Count()
builder method to count the number
of documents in an aggregation pipeline.
Example
The following example performs the following actions:
Specifies a match stage to find documents with a
FinalGrade
value greater than80
Counts the number of documents that match the criteria
var filter = Builders<Student> .Filter.Gt(s => s.FinalGrade, 80); var result = _myColl.Aggregate().Match(filter).Count(); Console.WriteLine("Number of documents with a final grade more than 80: " + result.First().Count);
Number of documents with a final grade more than 80: 4
Additional Information
To learn more about the operations mentioned, see the following guides:
API Documentation
To learn more about any of the methods or types discussed in this guide, see the following API Documentation: