Calculate product price counting with existing stock and price

* Closes: #20

api/db/inventary.go

@@ -52,7 +52,11 @@ func (d *DB) AddInventary(num int, inventary *Inventary) error {
 				query = query.Update("stock", gorm.Expr("stock + ?", *product.StockUpdate))
 			}
 			if product.Price != nil {
-				query = query.Update("price", *product.Price)
+				price, err := d.inventaryNewPrice(product)
+				if err != nil {
+					return err
+				}
+				query = query.Update("price", price)
 			}
 			err := query.Error
 			if err != nil {
@@ -64,6 +68,19 @@ func (d *DB) AddInventary(num int, inventary *Inventary) error {
 	})
 }
 
+func (d *DB) inventaryNewPrice(product InventaryProduct) (int, error) {
+	if product.StockUpdate == nil || *product.StockUpdate == 0 {
+		return *product.Price, nil
+	}
+
+	existingProduct, err := d.GetProduct(product.ProductCode)
+	if err != nil {
+		return 0, err
+	}
+	price := ((existingProduct.Price * existingProduct.Stock) + (*product.Price * *product.StockUpdate)) / (existingProduct.Stock + *product.StockUpdate)
+	return price, nil
+}
+
 func (d *DB) GetInventary(id int) (inventary Inventary, err error) {
 	err = d.db.Preload("Products.Product").
 		Preload(clause.Associations).

api/inventary_test.go

@@ -154,7 +154,8 @@ func TestInventaryUpdateBoth(t *testing.T) {
 	if product.Stock != testProduct.Stock+*inventary.Products[0].StockUpdate {
 		t.Error("Wrong stock:", product.Stock)
 	}
-	if product.Price != *inventary.Products[0].Price {
+	newPrice := ((price * updateStock) + (testProduct.Price * testProduct.Stock)) / (updateStock + testProduct.Stock)
+	if product.Price != newPrice {
 		t.Error("Wrong price:", product.Price)
 	}
 }