[#44] 팔로우 구현

- 팔로우 service 구현

src/main/java/com/social/instagram/domain/Follow.java

@@ -29,4 +29,11 @@ public Follow(String userId, String followId) {
         this.followId = followId;
     }
 
+    public static Follow from(String userId, String followId) {
+        return Follow.builder()
+                .userId(userId)
+                .followId(followId)
+                .build();
+    }
+
 }

src/main/java/com/social/instagram/repository/FollowRepository.java

@@ -0,0 +1,8 @@
+package com.social.instagram.repository;
+
+import com.social.instagram.domain.Follow;
+import org.springframework.data.jpa.repository.JpaRepository;
+
+public interface FollowRepository extends JpaRepository<Follow, Long> {
+
+}

src/main/java/com/social/instagram/service/FollowService.java

@@ -0,0 +1,20 @@
+package com.social.instagram.service;
+
+import com.social.instagram.domain.Follow;
+import com.social.instagram.repository.FollowRepository;
+import lombok.RequiredArgsConstructor;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+@Service
+@RequiredArgsConstructor
+public class FollowService {
+
+    private final FollowRepository followRepository;
+
+    @Transactional(rollbackFor = Exception.class)
+    public void followUserId(String userId, String followerId) {
+        followRepository.save(Follow.from(userId, followerId));
+    }
+
+}