feat(backend): users can follow/unfollow tags

backend/src/main/java/com/group1/programminglanguagesforum/Constants/EndpointConstants.java

@@ -49,6 +49,7 @@ public static class SparqlEndpoints {
     public static class TagEndpoints {
         public static final String BASE_PATH = "/tags";
         public static final String TAG_ID = BASE_PATH + "/{id}";
+        public static final String TAG_FOLLOW = BASE_PATH + "/{id}/follow";
         public static final String SEARCH = "/search" + BASE_PATH;
     }
 

backend/src/main/java/com/group1/programminglanguagesforum/Controllers/TagController.java

@@ -5,9 +5,16 @@
 import com.group1.programminglanguagesforum.DTOs.Responses.ErrorResponse;
 import com.group1.programminglanguagesforum.DTOs.Responses.GenericApiResponse;
 import com.group1.programminglanguagesforum.DTOs.Responses.GetTagDetailsResponseDto;
+import com.group1.programminglanguagesforum.DTOs.Responses.TagDto;
 import com.group1.programminglanguagesforum.DTOs.Responses.TagSearchResponseDto;
+import com.group1.programminglanguagesforum.Entities.User;
+import com.group1.programminglanguagesforum.Exceptions.ExceptionResponseHandler;
+import com.group1.programminglanguagesforum.Exceptions.UnauthorizedAccessException;
 import com.group1.programminglanguagesforum.Services.TagService;
+import com.group1.programminglanguagesforum.Services.UserContextService;
 import com.group1.programminglanguagesforum.Util.ApiResponseBuilder;
+
+import jakarta.persistence.EntityExistsException;
 import lombok.RequiredArgsConstructor;
 import org.springframework.data.domain.Page;
 import org.springframework.data.domain.PageRequest;
@@ -20,11 +27,15 @@
 import java.util.Arrays;
 import java.util.NoSuchElementException;
 
+
 @RestController
 @RequiredArgsConstructor
 @RequestMapping("/api/v1")
 public class TagController extends BaseController {
+
     private final TagService tagService;
+    private final UserContextService userContextService;
+
     @GetMapping(value = EndpointConstants.TagEndpoints.SEARCH)
     public ResponseEntity<GenericApiResponse<TagSearchResponseDto>> tagSearch(
             @RequestParam String q,
@@ -107,4 +118,37 @@ public ResponseEntity<GenericApiResponse<GetTagDetailsResponseDto>> createTag(@R
         }
 
     }
+
+    @PostMapping(value = EndpointConstants.TagEndpoints.TAG_FOLLOW)
+    public ResponseEntity<GenericApiResponse<TagDto>> postMethodName(@PathVariable(value = "id") Long tagId) {
+        try {
+            User user = userContextService.getCurrentUser();
+            TagDto tagDto = tagService.followTag(user, tagId);
+            GenericApiResponse<TagDto> response = ApiResponseBuilder.buildSuccessResponse(TagDto.class, "Tag followed successfully", 200, tagDto);
+            return buildResponse(response, HttpStatus.OK);
+
+        } catch (UnauthorizedAccessException e) {
+            return ExceptionResponseHandler.UnauthorizedAccessException(e);
+        } catch (NoSuchElementException e) {
+            return ExceptionResponseHandler.NoSuchElementException(e);
+        } catch (EntityExistsException e) {
+            return ExceptionResponseHandler.EntityExistsException(e);
+        }
+    }
+
+    @DeleteMapping(value = EndpointConstants.TagEndpoints.TAG_FOLLOW)
+    public ResponseEntity<GenericApiResponse<TagDto>> deleteMethodName(@PathVariable(value = "id") Long tagId) {
+        try {
+            User user = userContextService.getCurrentUser();
+            TagDto tagDto = tagService.unfollowTag(user, tagId);
+            GenericApiResponse<TagDto> response = ApiResponseBuilder.buildSuccessResponse(TagDto.class, "Tag unfollowed successfully", 200, tagDto);
+            return buildResponse(response, HttpStatus.OK);
+
+        } catch (UnauthorizedAccessException e) {
+            return ExceptionResponseHandler.UnauthorizedAccessException(e);
+        } catch (NoSuchElementException e) {
+            return ExceptionResponseHandler.NoSuchElementException(e);
+        }
+    }
+
 }

backend/src/main/java/com/group1/programminglanguagesforum/Entities/Tag.java

@@ -1,5 +1,8 @@
 package com.group1.programminglanguagesforum.Entities;
 
+import java.util.HashSet;
+import java.util.Set;
+
 import jakarta.persistence.*;
 import lombok.*;
 
@@ -18,6 +21,14 @@ public class Tag {
     private String wikidataId;
     private String tagName;
     private String tagDescription;
+
+    @ManyToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
+    @JoinTable(name = "USER_TAGS",
+            joinColumns = @JoinColumn(name = "tag_id"),
+            inverseJoinColumns = @JoinColumn(name = "user_id")
+    )
+    private Set<User> followers = new HashSet<>();
+
     public Tag(String wikidataId, String tagName, String tagDescription) {
         this.wikidataId = wikidataId;
         this.tagName = tagName;

backend/src/main/java/com/group1/programminglanguagesforum/Entities/User.java

@@ -57,6 +57,16 @@ public class User implements UserDetails {
     private int followingCount = 0;
     @Builder.Default
     private Long reputationPoints = 0L;
+
+    @ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
+    @JoinTable(
+            name = "USER_TAGS",
+            joinColumns = @JoinColumn(name = "user_id"),
+            inverseJoinColumns = @JoinColumn(name = "tag_id")
+    )
+    @Builder.Default
+    private Set<Tag> followedTags = new HashSet<>();
+
     @OneToMany(mappedBy = "askedBy", cascade = CascadeType.ALL, orphanRemoval = true)
     @Builder.Default
     private List<Question> questions = new ArrayList<>();

backend/src/main/java/com/group1/programminglanguagesforum/Exceptions/ExceptionResponseHandler.java

@@ -4,6 +4,9 @@
 import com.group1.programminglanguagesforum.DTOs.Responses.GenericApiResponse;
 import com.group1.programminglanguagesforum.DTOs.Responses.UserProfileResponseDto;
 import com.group1.programminglanguagesforum.Util.ApiResponseBuilder;
+
+import jakarta.persistence.EntityExistsException;
+
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
 
@@ -88,4 +91,43 @@ public static <T> ResponseEntity<GenericApiResponse<T>> NoSuchElementException(N
         );
 
     }
+
+    public static <T> ResponseEntity<GenericApiResponse<T>> IllegalArgumentException(IllegalArgumentException e) {
+        ErrorResponse errorResponse = ErrorResponse.builder()
+                .errorMessage(e.getMessage())
+                .stackTrace(Arrays.toString(e.getStackTrace()))
+                .build();
+
+        GenericApiResponse<T> response = ApiResponseBuilder.buildErrorResponse(
+                UserProfileResponseDto.class,
+                e.getMessage(),
+                HttpStatus.BAD_REQUEST.value(),
+                errorResponse
+        );
+
+        return new ResponseEntity<>(
+                response,
+                HttpStatus.valueOf(response.getStatus())
+        );
+    }
+
+    public static <T> ResponseEntity<GenericApiResponse<T>> EntityExistsException(EntityExistsException e) {
+        ErrorResponse errorResponse = ErrorResponse.builder()
+                .errorMessage(e.getMessage())
+                .stackTrace(Arrays.toString(e.getStackTrace()))
+                .build();
+
+        GenericApiResponse<T> response = ApiResponseBuilder.buildErrorResponse(
+                UserProfileResponseDto.class,
+                e.getMessage(),
+                HttpStatus.CONFLICT.value(),
+                errorResponse
+        );
+
+        return new ResponseEntity<>(
+                response,
+                HttpStatus.valueOf(response.getStatus())
+        );
+    }
+
 }

backend/src/main/java/com/group1/programminglanguagesforum/Services/TagService.java

@@ -5,6 +5,9 @@
 import com.group1.programminglanguagesforum.Entities.*;
 import com.group1.programminglanguagesforum.Repositories.QuestionRepository;
 import com.group1.programminglanguagesforum.Repositories.TagRepository;
+import com.group1.programminglanguagesforum.Repositories.UserRepository;
+
+import jakarta.persistence.EntityExistsException;
 import lombok.RequiredArgsConstructor;
 import org.modelmapper.ModelMapper;
 import org.springframework.data.domain.Page;
@@ -21,6 +24,7 @@ public class TagService {
     private final TagRepository tagRepository;
     private final ModelMapper modelMapper;
     private final QuestionRepository questionRepository;
+    private final UserRepository userRepository;
 
     public List<Tag> findAllByIdIn(List<Long> tagIds) {
         return tagRepository.findAllByIdIn(tagIds);
@@ -103,4 +107,49 @@ public Page<GetTagDetailsResponseDto> searchTags(String q, Pageable pageable) {
                 .tagType(getTagType(tag).toString())
                 .build());
     }
+
+    public TagDto followTag(User user, Long tagId) {
+
+        Optional<Tag> tag = tagRepository.findById(tagId);
+        if (tag.isEmpty()) {
+            throw new NoSuchElementException("Tag not found");
+        }
+
+        Tag tagEntity = tag.get();
+
+        if (user.getFollowedTags().stream().anyMatch(t -> t.getId().equals(tagId))) {
+            throw new EntityExistsException("User already follows this tag");
+        }
+
+        user.getFollowedTags().add(tagEntity);
+        userRepository.save(user);
+
+        return TagDto.builder()
+                .id(tagEntity.getId())
+                .name(tagEntity.getTagName())
+                .build();
+    }
+
+    public TagDto unfollowTag(User user, Long tagId) {
+
+        Optional<Tag> tag = tagRepository.findById(tagId);
+        if (tag.isEmpty()) {
+            throw new NoSuchElementException("Tag not found");
+        }
+
+        Tag tagEntity = tag.get();
+
+        if (!user.getFollowedTags().stream().anyMatch(t -> t.getId().equals(tagId))) {
+            throw new NoSuchElementException("User does not follow this tag");
+        }
+
+        user.getFollowedTags().removeIf(t -> t.getId().equals(tagId));
+        userRepository.save(user);
+
+        return TagDto.builder()
+                .id(tagEntity.getId())
+                .name(tagEntity.getTagName())
+                .build();
+    }
+
 }

backend/src/test/java/com/group1/programminglanguagesforum/Services/TagServiceTests.java

@@ -67,7 +67,7 @@ void testCreateTag() {
                 .build();
 
         // Mock the Tag returned by the repository save method
-        Tag savedTag = new Tag(1L, null, "New Tag", "Tag description");
+        Tag savedTag = new Tag(1L, null, "New Tag", "Tag description", null);
 
         // Mock tagRepository behavior
         when(tagRepository.save(any(Tag.class))).thenReturn(savedTag);
@@ -90,7 +90,7 @@ void testCreateTag() {
     void testGetTagDetails_Success() {
         Long tagId = 1L;
 
-        Tag mockTag = new Tag(1L, null, "Tag1", "Description1");
+        Tag mockTag = new Tag(1L, null, "Tag1", "Description1", null);
         List<Question> mockQuestions = Arrays.asList(
                 new Question(1L, "Question1", "Body1", DifficultyLevel.EASY, 0L, 0L, null, null, null, null, null,null),
                 new Question(2L, "Question2", "Body2", DifficultyLevel.MEDIUM, 0L, 0L, null, null, null, null, null,null));