DOCSP-13864 count documents usage example (#21)

* added count documents usage example

Author: kyuan-mongodb

source/includes/usage-examples/code-snippets/count.go

@@ -0,0 +1,51 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"os"
+
+	"go.mongodb.org/mongo-driver/bson"
+	"go.mongodb.org/mongo-driver/mongo"
+	"go.mongodb.org/mongo-driver/mongo/options"
+)
+
+func main() {
+	var uri string
+	if uri = os.Getenv("MONGODB_URI"); uri == "" {
+		log.Fatal("You must set your 'MONGODB_URI' environmental variable. See\n\t https://docs.mongodb.com/drivers/go/current/usage-examples/")
+	}
+
+	client, err := mongo.Connect(context.TODO(), options.Client().ApplyURI(uri))
+	if err != nil {
+		panic(err)
+	}
+	defer func() {
+		if err = client.Disconnect(context.TODO()); err != nil {
+			panic(err)
+		}
+	}()
+
+	// begin countDocuments
+	coll := client.Database("sample_mflix").Collection("movies")
+	filter := bson.D{{"countries", "China"}}
+
+	estCount, estCountErr := coll.EstimatedDocumentCount(context.TODO())
+	count, countErr := coll.CountDocuments(context.TODO(), filter)
+	// end countDocuments
+
+	if estCountErr != nil {
+		panic(estCountErr)
+	}
+	if countErr != nil {
+		panic(countErr)
+	}
+
+	// When you run this file, it should print:
+	// Estimated number of documents in the movies collection: 23541
+	// Number of movies from China: 303
+	
+	fmt.Printf("Estimated number of documents in the movies collection: %d\n", estCount)
+	fmt.Printf("Number of movies from China: %d\n", count)
+}

source/usage-examples.txt

@@ -15,12 +15,12 @@ Usage Examples
    /usage-examples/find-operations
    /usage-examples/update-operations
    /usage-examples/delete-operations
-
+   /usage-examples/count
+   
 ..
    /usage-examples/insert-operations
    /usage-examples/bulkWrite
    /usage-examples/watch
-   /usage-examples/count
    /usage-examples/distinct
    /usage-examples/command
 

source/usage-examples/count.txt

@@ -3,3 +3,49 @@ Count Documents
 ===============
 
 .. default-domain:: mongodb
+
+You can get an approximation on the number of documents in a
+collection using the ``EstimatedDocumentCount()`` method and an exact
+number of documents in a collection using the ``CountDocuments()``
+method.
+
+The following example uses the ``EstimatedDocumentCount()`` method to
+count the number of documents in the ``movies`` collection. Then, the
+example uses the ``CountDocuments()`` method to count the number of
+documents in the ``movies`` collection where the ``countries`` field
+contains "China".
+
+.. include:: /includes/usage-examples/run-example-tip.rst
+
+.. literalinclude:: /includes/usage-examples/code-snippets/count.go
+   :start-after: begin countDocuments
+   :end-before: end countDocuments
+   :emphasize-lines: 4-5
+   :language: go
+   :dedent:
+
+Click `here <{+example+}/count.go>`__ to see a fully runnable example.
+
+Expected Result
+---------------
+
+After running the preceding code snippet, you should see the following:
+
+- There are about ``23541`` documents in the ``movies`` collection  
+- There are ``303`` documents in the ``movies`` collection from "China"
+
+.. note::
+
+    The exact number of documents may vary depending on your data set.
+
+Additional Information
+----------------------
+
+For more information on counting documents, see our guide on
+<TODO: Counting Documents>. 
+
+API Documentation
+~~~~~~~~~~~~~~~~~
+
+- `CountDocuments() <{+godocs+}/mongo#Collection.CountDocuments>`__
+- `EstimatedDocumentCount() <{+godocs+}/mongo#Collection.EstimatedDocumentCount>`__