feat: improvements in structures

domain/entity/book_fixture.go

@@ -0,0 +1,16 @@
+package entity
+
+import (
+	"time"
+)
+
+func NewFixtureBook() *Book {
+	return &Book{
+		ID:        NewID(),
+		Title:     "I Am Ozzy",
+		Author:    "Ozzy Osbourne",
+		Pages:     294,
+		Quantity:  1,
+		CreatedAt: time.Now(),
+	}
+}