refactor(follow): 팔로우 영속화 (#7)

* refactor(follow): Follow 객체 생성

* refactor(follow): Follow 영속화

* refactor(follow): Followers 제거

* refactor(follow): 트랜잭션 지정

src/main/java/com/demo/feedsystemdesign/follow/domain/Follow.java

@@ -0,0 +1,27 @@
+package com.demo.feedsystemdesign.follow.domain;
+
+import com.demo.feedsystemdesign.common.BaseTimeEntity;
+import jakarta.persistence.*;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+
+@Entity
+@Getter
+@NoArgsConstructor
+public class Follow extends BaseTimeEntity {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    @Column(name = "follow_id")
+    private Long id;
+
+    private long sourceId;
+    private long targetId;
+
+    public Follow(long sourceId, long targetId) {
+        this.sourceId = sourceId;
+        this.targetId = targetId;
+    }
+
+}
+

src/main/java/com/demo/feedsystemdesign/follow/domain/FollowRepository.java

@@ -0,0 +1,13 @@
+package com.demo.feedsystemdesign.follow.domain;
+
+import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.stereotype.Repository;
+
+import java.util.List;
+
+@Repository
+public interface FollowRepository extends JpaRepository<Follow, Long> {
+    List<Follow> findAllByTargetId(Long targetId);
+
+    List<Follow> findAllBySourceId(Long sourceId);
+}

src/main/java/com/demo/feedsystemdesign/follow/service/FollowService.java

@@ -1,45 +1,43 @@
 package com.demo.feedsystemdesign.follow.service;
 
 import com.demo.feedsystemdesign.common.exception.NotFoundException;
-import com.demo.feedsystemdesign.follow.domain.Followers;
+import com.demo.feedsystemdesign.follow.domain.Follow;
+import com.demo.feedsystemdesign.follow.domain.FollowRepository;
 import com.demo.feedsystemdesign.user.domain.UserRepository;
+import jakarta.transaction.Transactional;
+import lombok.RequiredArgsConstructor;
 import org.springframework.stereotype.Service;
 
-import java.util.HashMap;
 import java.util.List;
-import java.util.Map;
 
 import static com.demo.feedsystemdesign.common.exception.ErrorCode.USER_NOT_FOUND;
 
+@RequiredArgsConstructor
 @Service
 public class FollowService {
 
     private final UserRepository userRepository;
-    private final Map<Long, Followers> followersRepository = new HashMap<>();
+    private final FollowRepository followRepository;
 
-    public FollowService(UserRepository userRepository) {
-        this.userRepository = userRepository;
-    }
-
-    public void follow(long userId, long subjectId) {
-        validateExists(userId);
-        validateExists(subjectId);
+    @Transactional
+    public void follow(long sourceId, long targetId) {
+        validateExists(sourceId);
+        validateExists(targetId);
 
-        if (followersRepository.get(subjectId) == null) {
-            followersRepository.put(subjectId, new Followers(subjectId));
-        }
-        Followers followers = followersRepository.get(subjectId);
-        followers.add(userId);
+        followRepository.save(new Follow(sourceId, targetId));
     }
 
     public List<Long> getFollowers(long userId) {
-        return followersRepository.get(userId).findAll();
+        return followRepository.findAllByTargetId(userId)
+                .stream()
+                .map(Follow::getSourceId)
+                .toList();
     }
 
     public List<Long> getFollowings(long userId) {
-        return followersRepository.values().stream()
-                .filter(followers -> followers.contains(userId))
-                .map(Followers::getOwnerId)
+        return followRepository.findAllBySourceId(userId)
+                .stream()
+                .map(Follow::getTargetId)
                 .toList();
     }