[REFACTOR] 장바구니 조회 기능 개선

src/main/kotlin/com/dh/baro/cart/application/CartFacade.kt

@@ -1,22 +1,42 @@
 package com.dh.baro.cart.application
 
-import com.dh.baro.cart.domain.CartItem
 import com.dh.baro.cart.domain.CartService
 import com.dh.baro.cart.presentation.dto.AddItemRequest
+import com.dh.baro.core.ErrorMessage
+import com.dh.baro.identity.domain.service.UserService
+import com.dh.baro.product.domain.service.ProductQueryService
 import org.springframework.stereotype.Service
+import org.springframework.transaction.annotation.Transactional
 
 @Service
 class CartFacade(
     private val cartService: CartService,
+    private val userService: UserService,
+    private val productQueryService: ProductQueryService,
 ) {
 
-    fun getCartItems(userId: Long): List<CartItem> =
-        cartService.getItems(userId)
+    @Transactional(readOnly = true)
+    fun getCartItems(userId: Long): List<CartItemBundle> {
+        userService.checkUserExists(userId)
+        val cartItems = cartService.getItems(userId)
+        val productIds = cartItems.map { it.productId }
+        val products = productQueryService.getAllByIds(productIds).associateBy { it.id }
 
+        return cartItems.map { cartItem ->
+            val product = products[cartItem.productId]
+                ?: throw IllegalStateException(ErrorMessage.PRODUCT_NOT_FOUND.format(cartItem.productId))
+
+            CartItemBundle(cartItem, product)
+        }
+    }
+
+    @Transactional
     fun addItem(userId: Long, request: AddItemRequest) {
+        userService.checkUserExists(userId)
+        productQueryService.checkProductsExists(listOf(request.productId))
         cartService.addItem(
             userId = userId,
-            productId = request.productId.toLong(),
+            productId = request.productId,
             quantity = request.quantity,
         )
     }

src/main/kotlin/com/dh/baro/cart/application/CartItemBundle.kt

@@ -0,0 +1,9 @@
+package com.dh.baro.cart.application
+
+import com.dh.baro.cart.domain.CartItem
+import com.dh.baro.product.domain.Product
+
+data class CartItemBundle(
+    val cartItem: CartItem,
+    val product: Product,
+)

src/main/kotlin/com/dh/baro/cart/domain/CartItem.kt

@@ -2,8 +2,6 @@ package com.dh.baro.cart.domain
 
 import com.dh.baro.core.BaseTimeEntity
 import com.dh.baro.core.IdGenerator
-import com.dh.baro.identity.domain.User
-import com.dh.baro.product.domain.Product
 import jakarta.persistence.*
 
 @Entity
@@ -16,13 +14,11 @@ class CartItem(
     @Column(name = "id")
     val id: Long,
 
-    @ManyToOne(fetch = FetchType.LAZY)
-    @JoinColumn(name = "user_id", nullable = false)
-    val user: User,
+    @Column(name = "user_id", nullable = false)
+    val userId: Long,
 
-    @ManyToOne(fetch = FetchType.LAZY)
-    @JoinColumn(name = "product_id", nullable = false)
-    val product: Product,
+    @Column(name = "product_id", nullable = false)
+    val productId: Long,
 
     @Column(name = "quantity", nullable = false)
     var quantity: Int = 1
@@ -39,11 +35,11 @@ class CartItem(
     }
 
     companion object {
-        fun newCartItem(user: User, product: Product, quantity: Int): CartItem {
+        fun newCartItem(userId: Long, productId: Long, quantity: Int): CartItem {
             return CartItem(
                 id = IdGenerator.generate(),
-                user = user,
-                product = product,
+                userId = userId,
+                productId = productId,
                 quantity = quantity,
             )
         }

src/main/kotlin/com/dh/baro/cart/domain/CartItemRepository.kt

@@ -1,11 +1,9 @@
 package com.dh.baro.cart.domain
 
-import org.springframework.data.jpa.repository.EntityGraph
 import org.springframework.data.jpa.repository.JpaRepository
 
 interface CartItemRepository : JpaRepository<CartItem, Long> {
 
-    @EntityGraph(attributePaths = ["product"])
     fun findByUserId(userId: Long): List<CartItem>
 
     fun findByUserIdAndProductId(userId: Long, productId: Long): CartItem?

src/main/kotlin/com/dh/baro/cart/domain/CartService.kt

@@ -1,17 +1,13 @@
 package com.dh.baro.cart.domain
 
 import com.dh.baro.core.ErrorMessage
-import com.dh.baro.identity.domain.repository.UserRepository
-import com.dh.baro.product.domain.repository.ProductRepository
-import org.springframework.data.repository.findByIdOrNull
+import org.springframework.dao.DataIntegrityViolationException
 import org.springframework.stereotype.Service
 import org.springframework.transaction.annotation.Transactional
 
 @Service
 class CartService(
     private val cartItemRepository: CartItemRepository,
-    private val userRepository: UserRepository,
-    private val productRepository: ProductRepository,
     private val cartPolicy: CartPolicy,
 ) {
 
@@ -23,20 +19,20 @@ class CartService(
     fun addItem(userId: Long, productId: Long, quantity: Int): CartItem {
         cartItemRepository.findByUserIdAndProductId(userId, productId)
             ?.also { it.addQuantity(quantity); return it }
-
         validateCartLimit(userId)
 
-        val user = userRepository.findByIdOrNull(userId)
-            ?: throw IllegalArgumentException(ErrorMessage.USER_NOT_FOUND.format(userId))
-        val product = productRepository.findByIdOrNull(productId)
-            ?: throw IllegalArgumentException(ErrorMessage.PRODUCT_NOT_FOUND.format(productId))
-
-        return cartItemRepository.save(CartItem.newCartItem(user, product, quantity))
+        return try {
+            cartItemRepository.save(CartItem.newCartItem(userId, productId, quantity))
+        } catch (e: DataIntegrityViolationException) {
+            val existingItem = cartItemRepository.findByUserIdAndProductId(userId, productId)?: throw e
+            existingItem.addQuantity(quantity)
+            existingItem
+        }
     }
 
     private fun validateCartLimit(userId: Long) {
-        val currentCnt = cartItemRepository.countByUserId(userId)
-        if (!cartPolicy.canAddMoreItems(currentCnt)) {
+        val currentItemCnt = cartItemRepository.countByUserId(userId)
+        if (!cartPolicy.canAddMoreItems(currentItemCnt)) {
             throw IllegalStateException(ErrorMessage.CART_ITEM_LIMIT_EXCEEDED.message)
         }
     }

src/main/kotlin/com/dh/baro/cart/presentation/CartController.kt

@@ -21,8 +21,10 @@ class CartController(
 
     @GetMapping
     @ResponseStatus(HttpStatus.OK)
-    override fun getCart(@CurrentUser userId: Long): CartResponse =
-        CartResponse.from(cartFacade.getCartItems(userId))
+    override fun getCart(@CurrentUser userId: Long): CartResponse {
+        val cartItemBundles = cartFacade.getCartItems(userId)
+        return CartResponse.from(cartItemBundles)
+    }
 
     @PostMapping("/items")
     @ResponseStatus(HttpStatus.CREATED)
@@ -35,14 +37,14 @@ class CartController(
     @ResponseStatus(HttpStatus.NO_CONTENT)
     override fun updateQuantity(
         @CurrentUser userId: Long,
-        @PathVariable itemId: String,
+        @PathVariable itemId: Long,
         @Valid @RequestBody request: UpdateQuantityRequest,
-    ) = cartFacade.updateQuantity(userId, itemId.toLong(), request.quantity)
+    ) = cartFacade.updateQuantity(userId, itemId, request.quantity)
 
     @DeleteMapping("/items/{itemId}")
     @ResponseStatus(HttpStatus.NO_CONTENT)
     override fun removeItem(
         @CurrentUser userId: Long,
-        @PathVariable itemId: String,
-    ) = cartFacade.removeItem(userId, itemId.toLong())
+        @PathVariable itemId: Long,
+    ) = cartFacade.removeItem(userId, itemId)
 }

src/main/kotlin/com/dh/baro/cart/presentation/dto/AddItemRequest.kt

@@ -1,11 +1,11 @@
 package com.dh.baro.cart.presentation.dto
 
 import jakarta.validation.constraints.Min
-import jakarta.validation.constraints.NotBlank
+import jakarta.validation.constraints.NotNull
 
 data class AddItemRequest(
-    @field:NotBlank(message = "상품 ID를 입력해주세요.")
-    val productId: String,
+    @field:NotNull(message = "상품 ID를 입력해주세요.")
+    val productId: Long,
     @field:Min(value = 1, message = "수량은 1개 이상이어야 합니다.")
     val quantity: Int,
 )

src/main/kotlin/com/dh/baro/cart/presentation/dto/CartItemResponse.kt

@@ -1,13 +1,41 @@
 package com.dh.baro.cart.presentation.dto
 
+import com.dh.baro.cart.application.CartItemBundle
+import com.dh.baro.core.LongToStringSerializer
+import com.fasterxml.jackson.databind.annotation.JsonSerialize
 import java.math.BigDecimal
+import java.math.RoundingMode
 
 data class CartItemResponse(
-    val itemId: String,
-    val productId: String,
+    @JsonSerialize(using = LongToStringSerializer::class)
+    val itemId: Long,
+    @JsonSerialize(using = LongToStringSerializer::class)
+    val productId: Long,
     val productName: String,
     val productThumbnailUrl: String?,
     val price: BigDecimal,
     val quantity: Int,
     val subtotal: BigDecimal,
-)
+) {
+
+    companion object {
+        private const val SCALE_NONE = 0
+
+        fun from(bundle: CartItemBundle): CartItemResponse {
+            val cartItem = bundle.cartItem
+            val product = bundle.product
+
+            return CartItemResponse(
+                itemId = cartItem.id,
+                productId = product.id,
+                productName = product.getName(),
+                productThumbnailUrl = product.getThumbnailUrl(),
+                price = product.getPrice(),
+                quantity = cartItem.quantity,
+                subtotal = product.getPrice()
+                    .multiply(BigDecimal(cartItem.quantity))
+                    .setScale(SCALE_NONE, RoundingMode.HALF_UP)
+            )
+        }
+    }
+}

src/main/kotlin/com/dh/baro/cart/presentation/dto/CartResponse.kt

@@ -1,6 +1,6 @@
 package com.dh.baro.cart.presentation.dto
 
-import com.dh.baro.cart.domain.CartItem
+import com.dh.baro.cart.application.CartItemBundle
 import java.math.BigDecimal
 import java.math.RoundingMode
 
@@ -11,23 +11,16 @@ data class CartResponse(
     companion object {
         private const val SCALE_NONE = 0
 
-        fun from(cartItems: List<CartItem>): CartResponse {
-            val responses = cartItems.map { it.toResponse() }
-            val total = responses.fold(BigDecimal.ZERO) { sum, item -> sum + item.subtotal }
-                .setScale(SCALE_NONE, RoundingMode.HALF_UP)
-            return CartResponse(responses, total)
-        }
+        fun from(bundles: List<CartItemBundle>): CartResponse {
+            val items = bundles.map { bundle ->
+                CartItemResponse.from(bundle)
+            }
+
+            val totalPrice = items.fold(BigDecimal.ZERO) { sum, item ->
+                sum + item.subtotal
+            }.setScale(SCALE_NONE, RoundingMode.HALF_UP)
 
-        private fun CartItem.toResponse() = CartItemResponse(
-            itemId = id.toString(),
-            productId = product.id.toString(),
-            productName = product.getName(),
-            productThumbnailUrl = product.getThumbnailUrl(),
-            price = product.getPrice(),
-            quantity = quantity,
-            subtotal = product.getPrice()
-                .multiply(BigDecimal(quantity))
-                .setScale(SCALE_NONE, RoundingMode.HALF_UP)
-        )
+            return CartResponse(items, totalPrice)
+        }
     }
 }

src/main/kotlin/com/dh/baro/cart/presentation/swagger/CartSwagger.kt

@@ -121,7 +121,7 @@ interface CartSwagger {
     @PatchMapping("/items/{itemId}")
     fun updateQuantity(
         @Parameter(hidden = true) userId: Long,
-        @PathVariable itemId: String,
+        @PathVariable itemId: Long,
         @RequestBody request: UpdateQuantityRequest
     )
 
@@ -145,6 +145,6 @@ interface CartSwagger {
     @DeleteMapping("/items/{itemId}")
     fun removeItem(
         @Parameter(hidden = true) userId: Long,
-        @PathVariable itemId: String
+        @PathVariable itemId: Long
     )
 }

src/main/kotlin/com/dh/baro/core/Cursor.kt

@@ -1,5 +1,8 @@
 package com.dh.baro.core
 
+import com.fasterxml.jackson.databind.annotation.JsonSerialize
+
 class Cursor (
-    val id: String,
+    @JsonSerialize(using = LongToStringSerializer::class)
+    val id: Long,
 )

src/main/kotlin/com/dh/baro/core/LongToStringSerializer.kt

@@ -0,0 +1,15 @@
+package com.dh.baro.core
+
+import com.fasterxml.jackson.core.JsonGenerator
+import com.fasterxml.jackson.databind.JsonSerializer
+import com.fasterxml.jackson.databind.SerializerProvider
+
+class LongToStringSerializer : JsonSerializer<Long>() {
+    override fun serialize(value: Long?, gen: JsonGenerator, serializers: SerializerProvider) {
+        if (value != null) {
+            gen.writeString(value.toString())
+        } else {
+            gen.writeNull()
+        }
+    }
+}

src/main/kotlin/com/dh/baro/identity/presentation/dto/UserProfileResponse.kt

@@ -1,10 +1,13 @@
 package com.dh.baro.identity.presentation.dto
 
+import com.dh.baro.core.LongToStringSerializer
 import com.dh.baro.identity.domain.User
 import com.dh.baro.identity.domain.UserRole
+import com.fasterxml.jackson.databind.annotation.JsonSerialize
 
 data class UserProfileResponse(
-    val id: String,
+    @JsonSerialize(using = LongToStringSerializer::class)
+    val id: Long,
     val name: String,
     val email: String,
     val phoneNumber: String?,
@@ -14,7 +17,7 @@ data class UserProfileResponse(
 
     companion object {
         fun from(user: User) = UserProfileResponse(
-            id = user.id.toString(),
+            id = user.id,
             name = user.getName(),
             email = user.getEmail(),
             phoneNumber = user.getPhoneNumber(),