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In this lesson we add a GET route that returns a single product by its ID as JSON. The following sections present the product model, helper functions to fetch all products and a single product from the database, the HTTP handler, and how to register the routes in your application. Code examples assume a standard Go web app using database/sql and gorilla/mux. Key topics covered:
  • product model and querying all products
  • parameterized single-row query to fetch a product by ID
  • HTTP handler for GET /product/
  • route registration
Useful references:

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Product model and getProducts (fetch all)

Define the product struct and a helper function to return all products from the database. This function uses a simple SELECT and scans the rows into a slice of products. 
import (
    "database/sql"
)

type product struct {
    ID       int     `json:"id"`
    Name     string  `json:"name"`
    Quantity int     `json:"quantity"`
    Price    float64 `json:"price"`
}

func getProducts(db *sql.DB) ([]product, error) {
    query := "SELECT id, name, quantity, price FROM products"
    rows, err := db.Query(query)
    if err != nil {
        return nil, err
    }
    defer rows.Close()

    products := []product{}
    for rows.Next() {
        var p product
        err := rows.Scan(&p.ID, &p.Name, &p.Quantity, &p.Price)
        if err != nil {
            return nil, err
        }
        products = append(products, p)
    }

    if err = rows.Err(); err != nil {
        return nil, err
    }

    return products, nil
}
Example response returned by /products (for reference): 
[
  {
    "id": 1,
    "name": "chair",
    "quantity": 100,
    "price": 200
  },
  {
    "id": 2,
    "name": "desk",
    "quantity": 800,
    "price": 600
  }
]

​
Register routes

Register the routes for listing all products and fetching a single product by ID. This assumes your App struct includes Router *mux.Router and DB *sql.DB, and that the handler methods are defined on App. 
func (app *App) handleRoutes() {
    app.Router.HandleFunc("/products", app.getProducts).Methods("GET")
    app.Router.HandleFunc("/product/{id}", app.getProduct).Methods("GET")
}
Table — Routes overview
RouteDescriptionResponse
GET /productsReturns all products200 OK, JSON array
GET /product/Returns one product by ID200 OK JSON or 400/404/500 on error

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Fetch a single product (model method)

Create a method on product that populates the struct fields based on the product ID. Always use parameterized queries to avoid SQL injection. The placeholder token depends on your SQL driver (e.g., ? for MySQL, $1 for Postgres). The example below uses ?—adjust for your driver accordingly. 
import (
    "database/sql"
)

func (p *product) getProduct(db *sql.DB) error {
    // Use a parameterized query. Placeholder depends on the driver.
    query := "SELECT name, quantity, price FROM products WHERE id = ?"
    row := db.QueryRow(query, p.ID)
    err := row.Scan(&p.Name, &p.Quantity, &p.Price)
    if err != nil {
        return err
    }
    return nil
}
When using database/sql, prefer QueryRow with arguments rather than formatting values into the SQL string. This avoids SQL injection and is safer.

​
Handler: GET /product/

The handler extracts the {id} path variable using mux.Vars, converts it to an integer with strconv.Atoi, constructs a product{ID: key}, calls the model method, and returns the appropriate HTTP response:
  • 400 Bad Request when the path parameter is missing or not an integer.
  • 404 Not Found when no matching row exists.
  • 500 Internal Server Error for other DB errors.
  • 200 OK with the product JSON on success.
import (
    "database/sql"
    "errors"
    "net/http"
    "strconv"

    "github.com/gorilla/mux"
)

func (app *App) getProduct(w http.ResponseWriter, r *http.Request) {
    vars := mux.Vars(r)
    keyStr, ok := vars["id"]
    if !ok {
        sendError(w, http.StatusBadRequest, "invalid product ID")
        return
    }

    key, err := strconv.Atoi(keyStr)
    if err != nil {
        sendError(w, http.StatusBadRequest, "invalid product ID")
        return
    }

    p := product{ID: key}
    err = p.getProduct(app.DB)
    if err != nil {
        if errors.Is(err, sql.ErrNoRows) {
            sendError(w, http.StatusNotFound, "Product not found")
        } else {
            sendError(w, http.StatusInternalServerError, err.Error())
        }
        return
    }

    sendResponse(w, http.StatusOK, p)
}
Notes:
  • sendError and sendResponse are utility helpers that should write JSON responses and set the proper Content-Type and status code.
  • The getProduct method uses a pointer receiver so the handler can return the populated product value directly.
Example implementations for utility functions (not required, but typically like): 
// sendError writes {"error": "<message>"} with the given status code.
func sendError(w http.ResponseWriter, code int, message string) {
    w.Header().Set("Content-Type", "application/json")
    w.WriteHeader(code)
    fmt.Fprintf(w, `{"error":"%s"}`, message)
}

// sendResponse writes the value as JSON with the given status code.
func sendResponse(w http.ResponseWriter, code int, v interface{}) {
    w.Header().Set("Content-Type", "application/json")
    w.WriteHeader(code)
    json.NewEncoder(w).Encode(v)
}

​
Example requests and responses

  • GET a valid product:
    • Request: GET http://localhost:10000/product/1
    • Response 200: 
      {
  "id": 1,
  "name": "chair",
  "quantity": 100,
  "price": 200
}
  • GET with an invalid integer ID:
    • Request: GET http://localhost:10000/product/abc
    • Response 400: 
      {
  "error": "invalid product ID"
}
  • GET a non-existent product ID:
    • Request: GET http://localhost:10000/product/10
    • Response 404: 
      {
  "error": "Product not found"
}
Summary With the model method, handler, and routes above wired into your application, you will have:
  • /products — returns all products as JSON
  • /product/ — retrieves a single product by ID with proper error handling and secure parameterized queries
For more best practices and production considerations, see:

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