Specify Which Fields to Return
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Overview
In this guide, you can learn how to specify which fields to return in a document.
Sample Data
The examples in this guide use the following Course
struct as a model for documents
in the courses
collection:
type Course struct { Title string `bson:"title,omitempty"` CourseId string `bson:"course_id,omitempty"` Enrollment int32 `bson:"enrollment,omitempty"` }
The omitempty
struct tag directs the
driver to exclude fields when unmarshalling based on your projection
specification.
To run the examples in this guide, load the sample data into the
db.courses
collection with the following snippet:
coll := client.Database("db").Collection("courses") docs := []interface{}{ Course{Title: "Primate Behavior", CourseId: "PSY2030", Enrollment: 40}, Course{Title: "Revolution and Reform", CourseId: "HIST3080", Enrollment: 12}, } 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 contains a description of a university course that
includes the course title, course ID, and maximum enrollment, corresponding to
the title
, course_id
, and enrollment
fields in each document.
Projection
A projection specifies which fields to return in matched documents. The
projection document contains field names with a 1
to include the
corresponding field or 0
to exclude it. If you are using an aggregation framework,
you can also specify a projection to include newly computed fields.
You can specify a projection by passing a projection document to the SetProjection()
method. The following operations take an options object as a parameter:
Find()
FindOne()
FindOneAndDelete()
FindOneAndReplace()
FindOneAndUpdate()
Tip
If you don't specify a projection, the operation returns all the fields in matched documents.
Exclude a Field
To exclude a field, pass the field you want to exclude with a 0
to
the SetProjection()
method. The driver includes all fields that are
not explicitly excluded in the projection document, if you specify any
fields to exclude.
Example
The following example excludes the course_id
and enrollment
fields from the matched documents returned by the Find()
method:
filter := bson.D{} opts := options.Find().SetProjection(bson.D{{"course_id", 0}, {"enrollment", 0}}) cursor, err := coll.Find(context.TODO(), filter, opts) if err != nil { panic(err) } var results []Course 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)) }
Include a Field
To include a field, pass the field you want to include with a 1
to
the SetProjection()
method. The driver excludes all fields that are
not explicitly included in the projection document, if you specify any
fields to include.
Example
The following example includes only the title
and enrollment
fields
from the matched documents returned by the Find()
method:
filter := bson.D{} opts := options.Find().SetProjection(bson.D{{"title", 1}, {"enrollment", 1}}) cursor, err := coll.Find(context.TODO(), filter, opts) if err != nil { panic(err) } var results []Course 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)) }
Aggregation
You can also create a $project stage to specify a projection in an aggregation pipeline.
Example
The following example includes only the title
and course_id
fields
from the matched documents returned by the Aggregate()
method:
projectStage := bson.D{{"$project", bson.D{{"title", 1}, {"course_id", 1}}}} cursor, err := coll.Aggregate(context.TODO(), mongo.Pipeline{projectStage}) if err != nil { panic(err) } var results []Course 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)) }
Additional Information
To learn more about the operations mentioned, see the following guides:
To learn about projecting text scores from your text search, see Search Text.
API Documentation
To learn more about any of the methods or types discussed in this guide, see the following API Documentation: