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Struct-Based Design in Go

bookStructs as API Models

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When building backend APIs in Go, you often need to model the data that flows through your endpoints. Structs are the primary way to define the shape of both requests and responses. By using structs, you can clearly describe the expected fields, their types, and how they map to JSON, which is the most common data format for APIs. This is achieved by attaching field tags to struct fields, allowing Go's standard library to automatically serialize and deserialize data between Go structs and JSON objects.

main.go

main.go

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package main

import (
	"encoding/json"
	"fmt"
)

// Order models an API request/response for an e-commerce order.
type Order struct {
	ID         int      `json:"id"`
	Customer   string   `json:"customer"`
	Items      []string `json:"items"`
	UnitPrices []float64 `json:"unit_prices"`
	Quantities []int    `json:"quantities"`
}

// TotalPrice calculates the total price for the order.
func (o *Order) TotalPrice() float64 {
	total := 0.0
	for i := range o.Items {
		if i < len(o.UnitPrices) && i < len(o.Quantities) {
			total += o.UnitPrices[i] * float64(o.Quantities[i])
		}
	}
	return total
}

func main() {
	order := Order{
		ID:         101,
		Customer:   "Alice",
		Items:      []string{"Book", "Pen"},
		UnitPrices: []float64{12.5, 1.5},
		Quantities: []int{2, 5},
	}

	// Serialize to JSON
	data, _ := json.Marshal(order)
	fmt.Println("Serialized JSON:", string(data))

	// Calculate total price
	fmt.Printf("Total price: %.2f\n", order.TotalPrice())
}

Struct tags in Go are string annotations that appear after field declarations, enclosed in backticks. For API models, the most common use is to control how fields are mapped to JSON keys using the json tag. When you marshal or unmarshal JSON, the encoding/json package reads these tags to match struct fields with JSON object properties.

Best practices for API model design include:

  • Use exported field names (capitalized) so that encoding/json can access them;
  • Keep struct definitions focused on the data actually needed by the API;
  • Use clear, consistent JSON tag names that match your API specification;
  • Avoid including business logic in API model structs, except for utility methods like total calculations;
  • Document any non-obvious field mappings or required fields.
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Which of the following best describes the role of struct tags in Go API model design?

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