Retrieve Data

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Overview

Sample Data
Find Operations
Find All Documents
Find One Document
Modify Behavior
Aggregation Operations
Aggregation
Modify Behavior
Additional Information
API Documentation

Overview

In this guide, you can learn how to retrieve data from your MongoDB collections using read operations.

Read operations allow you to do the following:

Retrieve documents from your collections by using find operations
Perform transformations on documents in your collections by using aggregation operations

Sample Data

The examples in this guide use the following Tea struct as a model for documents in the tea collection:

type Tea struct {
	Item        string    `bson:"item,omitempty"`
	Rating      int32     `bson:"rating,omitempty"`
	DateOrdered time.Time `bson:"date_ordered,omitempty"`
}

To run the examples in this guide, load these documents into the tea collection in the db database by using the following snippet:

coll := client.Database("db").Collection("tea")
docs := []interface{}{
	Tea{Item: "Masala", Rating: 10, DateOrdered: time.Date(2009, 11, 17, 0, 0, 0, 0, time.Local)},
	Tea{Item: "Sencha", Rating: 7, DateOrdered: time.Date(2009, 11, 18, 0, 0, 0, 0, time.Local)},
	Tea{Item: "Masala", Rating: 9, DateOrdered: time.Date(2009, 11, 12, 0, 0, 0, 0, time.Local)},
	Tea{Item: "Masala", Rating: 8, DateOrdered: time.Date(2009, 12, 1, 0, 0, 0, 0, time.Local)},
	Tea{Item: "Sencha", Rating: 10, DateOrdered: time.Date(2009, 12, 17, 0, 0, 0, 0, time.Local)},
	Tea{Item: "Hibiscus", Rating: 4, DateOrdered: time.Date(2009, 12, 18, 0, 0, 0, 0, time.Local)},
}
result, err := coll.InsertMany(context.TODO(), docs)

Tip

Nonexistent Databases and Collections

If the necessary database and collection don't exist when you perform a write operation, the server implicitly creates them.

Each document describes the tea variety a customer ordered, their rating, and the date of the order. These descriptions correspond to the item, rating, and date_ordered fields.

Find Operations

Use find operations to retrieve data from MongoDB. Find operations consist of the Find() and FindOne() methods.

Find All Documents

The Find() method expects you to pass a Context type and a query filter. The method returns all documents that match the filter as a Cursor type.

For an example that uses the Find() method, see the Find Example section of this page. To learn how to access data by using a cursor, see the Access Data From a Cursor guide.

Find One Document

The FindOne() method expects you to pass a Context type and a query filter. The method returns the first document that matches the filter as a SingleResult type.

For an example that uses the FindOne() method, see the Find One Example section of this page. For an example that uses FindOne() and queries by using a specific ObjectId value, see the Find One by ObjectId Example section of this page.

To learn how to access data from a SingleResult type, see Unmarshalling in the BSON guide.

Modify Behavior

You can modify the behavior of Find() and FindOne() by passing in a FindOptions and FindOneOptions type respectively. If you don't specify any options, the driver uses the default values for each option.

You can configure the commonly used options in both types with the following methods:

Method	Description
`SetCollation()`	The type of language collation to use when sorting results. Default: `nil`
`SetLimit()`	The maximum number of documents to return. Default: `0` This option is not available for `FindOneOptions`. The `FindOne()` method internally uses `SetLimit(-1)`.
`SetProjection()`	The fields to include in the returned documents. Default: `nil`
`SetSkip()`	The number of documents to skip. Default: `0`
`SetSort()`	The field and type of sort to order the matched documents. You can specify an ascending or descending sort. Default: none

Find Example

The following example passes a context, filter, and FindOptions to the Find() method, which performs the following actions:

Matches documents where the rating value is between 5 and 9 (exclusive)
Sorts the matched documents in ascending order by date_ordered

filter := bson.D{
	{"$and",
		bson.A{
			bson.D{{"rating", bson.D{{"$gt", 5}}}},
			bson.D{{"rating", bson.D{{"$lt", 9}}}},
		}},
}
sort := bson.D{{"date_ordered", 1}}
opts := options.Find().SetSort(sort)
// Retrieves documents that match the filter and prints them as structs
cursor, err := coll.Find(context.TODO(), filter, opts)
if err != nil {
	panic(err)
}
var results []Tea
if err = cursor.All(context.TODO(), &results); err != nil {
	panic(err)
}
for _, result := range results {
	res, _ := bson.MarshalExtJSON(result, false, false)
	fmt.Println(string(res))
}

{"item":"Sencha","rating":7,"date_ordered":"2009-11-18T05:00:00Z"}
{"item":"Masala","rating":8,"date_ordered":"2009-12-01T05:00:00Z"}

Find One Example

The following example passes a context, filter, and FindOneOptions to the FindOne() method, which performs the following actions:

Matches documents where the date_ordered value is on or before November 30, 2009
Skips the first two matched documents

filter := bson.D{{"date_ordered", bson.D{{"$lte", time.Date(2009, 11, 30, 0, 0, 0, 0, time.Local)}}}}
opts := options.FindOne().SetSkip(2)
// Retrieves a document that matches the filter and prints it as
// a struct
var result Tea
err = coll.FindOne(context.TODO(), filter, opts).Decode(&result)
if err != nil {
	if err == mongo.ErrNoDocuments {
		fmt.Println("No documents found")
	} else {
		panic(err)
	}
}
res, _ := bson.MarshalExtJSON(result, false, false)
fmt.Println(string(res))

{"item":"Masala","rating":9,"date_ordered":"2009-11-12T05:00:00Z"}

Find One by ObjectId Example

This example defines an id variable with a value of type ObjectId and uses id to specify a query filter. The filter matches a document with an _id field value that corresponds to the id variable. This example queries for the following document based on its _id value:

 {
     _id: ObjectId('65170b42b99efdd0b07d42de'),
     item: "Hibiscus",
     rating : 4,
     date_ordered : 2009-12-18T05:00:00.000+00:00
 }

The following code passes the filter and a FindOneOptions instance as parameters to the FindOne() method to perform the following actions:

Match the document with the specified ObjectId value
Project only the Item and Rating fields of the matched document

id, err := primitive.ObjectIDFromHex("65170b42b99efdd0b07d42de")
if err != nil {
	panic(err)
}
//  Creates a filter to match a document that has the specified
//  "_id" value
filter := bson.D{{"_id", id}}
opts := options.FindOne().SetProjection(bson.D{{"item", 1}, {"rating", 1}})
// Retrieves a document that matches the filter and prints it as
// a struct
var result Tea
err = coll.FindOne(context.TODO(), filter, opts).Decode(&result)
if err != nil {
	if err == mongo.ErrNoDocuments {
		fmt.Println("No documents found")
	} else {
		panic(err)
	}
}
res, _ := bson.MarshalExtJSON(result, false, false)
fmt.Println(string(res))

{"item":"Hibiscus","rating":4}

Note

The Go driver automatically generates a unique ObjectId value for each document's _id field, so your ObjectId value might differ from the preceding code example. For more information about the _id field, see the _id Field section of the Insert a Document page.

Aggregation Operations

Use aggregation operations to retrieve and transform data from MongoDB. Perform aggregation operations using the Aggregate() method.

Aggregation

The Aggregate() method expects you to pass a Context type and an aggregation pipeline. An aggregation pipeline defines how to transform data through stages. Some of the stages are matching documents, renaming fields, and grouping values.

The method returns the resulting documents in a Cursor type. If you omit the $match stage, the pipeline proceeds using all documents in the collection.

To learn how to access data in a cursor, see Access Data From a Cursor.

Modify Behavior

The Aggregate() method optionally takes an AggregateOptions type, which represents options you can use to modify its behavior. If you don't specify any options, the driver uses the default values for each option.

The AggregateOptions type allows you to configure options with the following methods:

Method	Description
`SetAllowDiskUse()`	Whether to write to temporary files. Default: `false`
`SetBatchSize()`	The number of documents to return in each batch. Default: none
`SetBypassDocumentValidation()`	Whether to allow the write to opt-out of document level validation. Default: `false`
`SetCollation()`	The type of language collation to use when sorting results. Default: `nil`
`SetMaxAwaitTime()`	The maximum amount of time for the server to wait on new documents to satisfy a tailable cursor query. Default: `nil`
`SetComment()`	An arbitrary string or document that allows you to trace the operation through the database profiler, currentOp, and logs. Default: `""`
`SetHint()`	The index to use to scan for documents to retrieve. Default: `nil`
`SetLet()`	Specifies parameters for the aggregate expression, which improves command readability by separating the variables from the query text. Default: none

Example

The following example passes a context and an aggregation pipeline that performs the following actions:

Groups reviews by item ordered
Calculates the average rating for each item

groupStage := bson.D{
	{"$group", bson.D{
		{"_id", "$item"},
		{"average", bson.D{
			{"$avg", "$rating"},
		}},
	}}}
cursor, err := coll.Aggregate(context.TODO(), mongo.Pipeline{groupStage})
if err != nil {
	panic(err)
}
// Prints the average "rating" for each item
var results []bson.M
if err = cursor.All(context.TODO(), &results); err != nil {
	panic(err)
}
for _, result := range results {
	fmt.Printf("%v had an average rating of %v \n", result["_id"], result["average"])
}

Sencha had an average rating of 8.5
Hibiscus had an average rating of 4
Masala had an average rating of 9

To learn more about how to construct an aggregation pipeline, see the MongoDB server manual page on Aggregation.

Additional Information

For runnable examples of the find operations, see the following usage examples:

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:

MongoDB Go Driver