Adds get by isbn route

Note: gin router has some very serious limitations which mean that
it does not work with some typical rest route patterns. See e.g.
gin-gonic/gin#1681

backend/main.go

@@ -28,6 +28,7 @@ func main() {
 			books.GET("/:id", BooksGet)
 			books.PATCH("/:id", BooksUpdate)
 			books.DELETE("/:id", BooksDelete)
+			// books.GET("/isbn/:isbn", BooksGetByISBN)  // gin router breaks for paths like this. See eg https://github.com/gin-gonic/gin/issues/1681. Consider switching framework
 		}
 	}
 

backend/models/book.go

@@ -12,7 +12,7 @@ type Book struct {
 	Title           string    `json:"title" binding:"required"`
 	Authors         string    `json:"authors" binding:"required"`
 	AverageRating   string    `json:"average_rating"`
-	ISBN            string    `json:"isbn" binding:"required"`
+	ISBN            string    `json:"isbn" binding:"required" gorm:"index"`
 	ISBN13          string    `json:"isbn_13"`
 	LanguageCode    string    `json:"language_code" binding:"required"`
 	NumPages        int       `json:"num_pages"`

backend/routes.go

@@ -77,3 +77,15 @@ func BooksDelete(c *gin.Context) {
 	db.Delete(&book)
 	c.JSON(http.StatusAccepted, gin.H{"data": true})
 }
+
+// BooksGetByISBN serves JSON response containing a single book by ISBN.
+func BooksGetByISBN(c *gin.Context) {
+	db := c.MustGet("db").(*gorm.DB)
+	book := models.Book{}
+	if err := db.Where("isbn = ?", c.Param("isbn")).First(&book).Error; err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "Record does not exist."})
+		return
+	}
+
+	c.JSON(http.StatusOK, gin.H{"data": book})
+}