Filtering what information is displayed from MongoDB in Go api

Issue

I am new to Go and React, both of which I am using for this mini project. Go is running a backend api using Mongodb.

I am fetching the user list from Mongo in Go, and then sending that to React, problem is Mongo is giving me all of the fields for the user (_id, password, and username), I only want username. I am not understanding how I can filter this and prevent all fields from being sent from Go to React.

JSON Output from Go API:

{
    "Success": true,
    "Data": [
        {
            "_id": "6205ac3d72c15c920a424608",
            "password": {
                "Subtype": 0,
                "Data": "removed for security"
            },
            "username": "removed for security"
        },
        {
            "_id": "6206b44afb8b044fdba76b8f",
            "password": {
                "Subtype": 0,
                "Data": "removed for security"
            },
            "username": "removed for security"
        }
    ]
}

Here is my Go code for the specified route:

// Route: Get Users, for getting a list of users
func RouteGetUsers(w http.ResponseWriter, r *http.Request, p httprouter.Params) {
    // Set content-type to JSON
    w.Header().Set("Content-Type", "application/json")

    type Response struct {
        Success bool     `json:"Success"`
        Data    []bson.M `json:"Data"`
    }

    // Load the env file
    err := godotenv.Load("variables.env")
    if err != nil {
        log.Fatal("Error loading .env file")
    }

    // Get Mongo DB environment variable
    uri := os.Getenv("MONGO_URI")
    if uri == "" {
        log.Fatal("You must set your 'MONGO_URI' environmental variable. See\n\t https://docs.mongodb.com/drivers/go/current/usage-examples/#environment-variable")
    }

    // Connect to Mongo Database
    client, err := mongo.Connect(context.TODO(), options.Client().ApplyURI(uri))
    if err != nil {
        panic(err)
    }

    // Close the database connection at the end of the function
    defer func() {
        if err := client.Disconnect(context.TODO()); err != nil {
            panic(err)
        }
    }()

    // Select the database name and collection name
    coll := client.Database("go_project1").Collection("users")

    // Query the database for the user list
    cursor, err := coll.Find(context.TODO(), bson.D{})
    // If no documents were found, send a response and return
    if err == mongo.ErrNoDocuments {
        fmt.Printf("No documents were found")
        return
    }

    // Setup a variable for the database results
    var results []bson.M

    // Send all database results to results variable
    if err = cursor.All(context.TODO(), &results); err != nil {
        panic(err)
    }

    // Setup a variable with the ResponseStandard struct
    response := Response{
        Success: true,
        Data:    results,
    }

    // Marshal into JSON
    responseJson, err := json.Marshal(response)
    if err != nil {
        fmt.Println(err)
    }

    // Send success response to user in JSON
    fmt.Fprintf(w, "%s\n", responseJson)
}

Solution

Use the projection option:

opts := options.Find().SetProjection(bson.D{{"username", 1}})
cursor, err := coll.Find(context.TODO(), bson.D{}, opts)

As second approach: Declare a type with the fields you want, and fetch to that type.

type Data struct {
   Username string `bson:"username" json:"username"`
}

...

var data []Data
if err = cursor.All(context.TODO(), &data); err != nil { ...

...

var response = struct {
    Success bool     `json:"Success"`
    Data    []Data   `json:"Data"`
}{
    true,
    data,
}
responseJson, err := json.Marshal(response)
...

A third approach: Filter the maps in the question:

for _, result := range results {
    for k := range result {
       if k != "username" {
          delete(result, k)
       }
    }
}

Answered By – Bayta Darell

Answer Checked By – Dawn Plyler (GoLangFix Volunteer)

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