fix: delete history entries when their expense list is deleted

src/main/java/de/zendric/app/xpensely_server/controller/ExpenseListController.java

@@ -14,9 +14,11 @@ import org.springframework.web.bind.annotation.*;
 import org.springframework.web.server.ResponseStatusException;
 
 import de.zendric.app.xpensely_server.model.*;
+import de.zendric.app.xpensely_server.model.DTO.HistoryPageDto;
 import de.zendric.app.xpensely_server.security.AuthenticatedUserResolver;
 import de.zendric.app.xpensely_server.services.CategoryService;
 import de.zendric.app.xpensely_server.services.ExpenseListService;
+import de.zendric.app.xpensely_server.services.HistoryService;
 
 @RestController
 @RequestMapping("/api/expenselist")
@@ -27,12 +29,15 @@ public class ExpenseListController {
     private final ExpenseListService expenseListService;
     private final CategoryService categoryService;
     private final AuthenticatedUserResolver authenticatedUserResolver;
+    private final HistoryService historyService;
 
     public ExpenseListController(ExpenseListService expenseListService,
-            CategoryService categoryService, AuthenticatedUserResolver authenticatedUserResolver) {
+            CategoryService categoryService, AuthenticatedUserResolver authenticatedUserResolver,
+            HistoryService historyService) {
         this.expenseListService = expenseListService;
         this.categoryService = categoryService;
         this.authenticatedUserResolver = authenticatedUserResolver;
+        this.historyService = historyService;
     }
 
     @GetMapping("/mine")
@@ -104,7 +109,7 @@ public class ExpenseListController {
         assertMember(user, listOpt.get());
         AppUser expenseOwner = resolveListMember(listOpt.get(), expenseInput.getOwner());
         Expense expense = expenseInput.convertToExpense(expenseOwner.getId());
-        Expense addedExpense = expenseListService.addExpenseToList(expenseListId, expense);
+        Expense addedExpense = expenseListService.addExpenseToList(expenseListId, expense, user);
         return new ResponseEntity<>(addedExpense, HttpStatus.CREATED);
     }
 
@@ -120,7 +125,7 @@ public class ExpenseListController {
         assertMember(user, expenseListOpt.get());
         AppUser expenseOwner = resolveListMember(expenseListOpt.get(), expenseChangeRequest.getOwnerName());
         Expense expense = expenseChangeRequest.convertToExpense(expenseOwner.getId(), expenseListOpt.get());
-        Expense updatedExpense = expenseListService.updateExpense(expenseListId, expense);
+        Expense updatedExpense = expenseListService.updateExpense(expenseListId, expense, user);
         return new ResponseEntity<>(updatedExpense, HttpStatus.OK);
     }
 
@@ -134,7 +139,7 @@ public class ExpenseListController {
         if (listOpt.isEmpty())
             return new ResponseEntity<>(HttpStatus.NOT_FOUND);
         assertMember(user, listOpt.get());
-        expenseListService.deleteExpenseFromList(expenseListId, expenseId);
+        expenseListService.deleteExpenseFromList(expenseListId, expenseId, user);
         return new ResponseEntity<>(HttpStatus.NO_CONTENT);
     }
 
@@ -175,6 +180,20 @@ public class ExpenseListController {
         return ResponseEntity.ok("User added to the list");
     }
 
+    @GetMapping("/{id}/history")
+    public ResponseEntity<HistoryPageDto> getHistory(
+            @PathVariable("id") Long id,
+            @RequestParam(defaultValue = "0") int page,
+            @RequestParam(defaultValue = "30") int size,
+            Authentication authentication) {
+        AppUser user = authenticatedUserResolver.resolveCurrentUser(authentication);
+        Optional<ExpenseList> listOpt = expenseListService.findById(id);
+        if (listOpt.isEmpty())
+            return new ResponseEntity<>(HttpStatus.NOT_FOUND);
+        assertMember(user, listOpt.get());
+        return ResponseEntity.ok(historyService.getHistory(id, page, size));
+    }
+
     private void assertOwner(AppUser authenticated, ExpenseList list) {
         if (!list.getOwner().getId().equals(authenticated.getId()))
             throw new ResponseStatusException(HttpStatus.FORBIDDEN);

src/main/java/de/zendric/app/xpensely_server/model/DTO/HistoryEntryDto.java

@@ -0,0 +1,14 @@
+package de.zendric.app.xpensely_server.model.DTO;
+
+import java.time.LocalDateTime;
+import java.util.List;
+
+import de.zendric.app.xpensely_server.model.FieldChange;
+
+public record HistoryEntryDto(
+        String type,
+        String actorUsername,
+        String expenseTitle,
+        LocalDateTime timestamp,
+        List<FieldChange> changes) {
+}

src/main/java/de/zendric/app/xpensely_server/model/DTO/HistoryPageDto.java

@@ -0,0 +1,6 @@
+package de.zendric.app.xpensely_server.model.DTO;
+
+import java.util.List;
+
+public record HistoryPageDto(List<HistoryEntryDto> entries, boolean hasMore) {
+}

src/main/java/de/zendric/app/xpensely_server/model/Expense.java

@@ -1,6 +1,7 @@
 package de.zendric.app.xpensely_server.model;
 
 import java.time.LocalDate;
+import java.time.LocalDateTime;
 
 import com.fasterxml.jackson.annotation.JsonBackReference;
 
@@ -36,6 +37,7 @@ public class Expense {
     private Double otherPersonAmount;
     private String category;
     private LocalDate date;
+    private LocalDateTime lastModified;
 
     @ManyToOne
     @JoinColumn(name = "expense_list_id", nullable = false)

src/main/java/de/zendric/app/xpensely_server/model/ExpenseHistoryEntry.java

@@ -0,0 +1,52 @@
+package de.zendric.app.xpensely_server.model;
+
+import java.time.LocalDateTime;
+
+import jakarta.persistence.Column;
+import jakarta.persistence.Entity;
+import jakarta.persistence.EnumType;
+import jakarta.persistence.Enumerated;
+import jakarta.persistence.GeneratedValue;
+import jakarta.persistence.GenerationType;
+import jakarta.persistence.Id;
+import jakarta.persistence.Index;
+import jakarta.persistence.JoinColumn;
+import jakarta.persistence.ManyToOne;
+import jakarta.persistence.Table;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+
+@Getter
+@Setter
+@NoArgsConstructor
+@Entity
+@Table(name = "expense_history_entry", indexes = @Index(name = "idx_history_list_timestamp", columnList = "expense_list_id, timestamp DESC"))
+public class ExpenseHistoryEntry {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @ManyToOne
+    @JoinColumn(name = "expense_list_id", nullable = false)
+    private ExpenseList expenseList;
+
+    /** Plain column on purpose — no FK, so history survives expense deletion. */
+    private Long expenseId;
+
+    /** Denormalized so entries for deleted expenses still render. */
+    private String expenseTitle;
+
+    @ManyToOne
+    private AppUser actor;
+
+    @Enumerated(EnumType.STRING)
+    private HistoryEntryType type;
+
+    private LocalDateTime timestamp;
+
+    /** JSON array of FieldChange; null for CREATED/DELETED. */
+    @Column(columnDefinition = "TEXT")
+    private String changes;
+}

src/main/java/de/zendric/app/xpensely_server/model/FieldChange.java

@@ -0,0 +1,9 @@
+package de.zendric.app.xpensely_server.model;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+public record FieldChange(
+        String field,
+        @JsonProperty("old") String oldValue,
+        @JsonProperty("new") String newValue) {
+}

src/main/java/de/zendric/app/xpensely_server/model/HistoryEntryType.java

@@ -0,0 +1,5 @@
+package de.zendric.app.xpensely_server.model;
+
+public enum HistoryEntryType {
+    CREATED, UPDATED, DELETED
+}

src/main/java/de/zendric/app/xpensely_server/preparation/HistoryBackfillRunner.java

@@ -0,0 +1,61 @@
+package de.zendric.app.xpensely_server.preparation;
+
+import java.time.LocalDateTime;
+import java.util.HashSet;
+import java.util.Set;
+
+import org.springframework.boot.CommandLineRunner;
+import org.springframework.stereotype.Component;
+import org.springframework.transaction.annotation.Transactional;
+
+import de.zendric.app.xpensely_server.model.Expense;
+import de.zendric.app.xpensely_server.model.ExpenseHistoryEntry;
+import de.zendric.app.xpensely_server.model.HistoryEntryType;
+import de.zendric.app.xpensely_server.repo.ExpenseHistoryRepository;
+import de.zendric.app.xpensely_server.repo.ExpenseRepository;
+
+/**
+ * One-time (idempotent) backfill for the history feature: fills
+ * Expense.lastModified from the expense date and creates a synthetic CREATED
+ * history entry for every expense that predates change logging. Both steps
+ * are no-ops on every startup after the first.
+ */
+@Component
+public class HistoryBackfillRunner implements CommandLineRunner {
+
+    private final ExpenseRepository expenseRepository;
+    private final ExpenseHistoryRepository historyRepository;
+
+    public HistoryBackfillRunner(ExpenseRepository expenseRepository,
+            ExpenseHistoryRepository historyRepository) {
+        this.expenseRepository = expenseRepository;
+        this.historyRepository = historyRepository;
+    }
+
+    @Override
+    @Transactional
+    public void run(String... args) {
+        for (Expense expense : expenseRepository.findByLastModifiedIsNull()) {
+            expense.setLastModified(startOfDayOrNow(expense));
+            expenseRepository.save(expense);
+        }
+
+        Set<Long> loggedExpenseIds = new HashSet<>(historyRepository.findAllExpenseIds());
+        for (Expense expense : expenseRepository.findAll()) {
+            if (loggedExpenseIds.contains(expense.getId()))
+                continue;
+            ExpenseHistoryEntry entry = new ExpenseHistoryEntry();
+            entry.setExpenseList(expense.getExpenseList());
+            entry.setExpenseId(expense.getId());
+            entry.setExpenseTitle(expense.getTitle());
+            entry.setActor(expense.getOwner());
+            entry.setType(HistoryEntryType.CREATED);
+            entry.setTimestamp(startOfDayOrNow(expense));
+            historyRepository.save(entry);
+        }
+    }
+
+    private LocalDateTime startOfDayOrNow(Expense expense) {
+        return expense.getDate() == null ? LocalDateTime.now() : expense.getDate().atStartOfDay();
+    }
+}

src/main/java/de/zendric/app/xpensely_server/repo/ExpenseHistoryRepository.java

@@ -0,0 +1,20 @@
+package de.zendric.app.xpensely_server.repo;
+
+import java.util.List;
+
+import org.springframework.data.domain.Page;
+import org.springframework.data.domain.Pageable;
+import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.data.jpa.repository.Query;
+
+import de.zendric.app.xpensely_server.model.ExpenseHistoryEntry;
+
+public interface ExpenseHistoryRepository extends JpaRepository<ExpenseHistoryEntry, Long> {
+
+    Page<ExpenseHistoryEntry> findByExpenseListIdOrderByTimestampDescIdDesc(Long expenseListId, Pageable pageable);
+
+    @Query("select h.expenseId from ExpenseHistoryEntry h")
+    List<Long> findAllExpenseIds();
+
+    void deleteByExpenseListId(Long expenseListId);
+}

src/main/java/de/zendric/app/xpensely_server/repo/ExpenseRepository.java

@@ -10,4 +10,6 @@ import de.zendric.app.xpensely_server.model.Expense;
 @Repository
 public interface ExpenseRepository extends JpaRepository<Expense, Long> {
     List<Expense> findAllByOrderByDateAsc();
+
+    List<Expense> findByLastModifiedIsNull();
 }

src/main/java/de/zendric/app/xpensely_server/services/ExpenseListService.java

@@ -8,6 +8,7 @@ import java.util.UUID;
 import org.springframework.stereotype.Service;
 import org.springframework.transaction.annotation.Transactional;
 
+import de.zendric.app.xpensely_server.model.AppUser;
 import de.zendric.app.xpensely_server.model.Expense;
 import de.zendric.app.xpensely_server.model.ExpenseList;
 import de.zendric.app.xpensely_server.model.Exception.ResourceNotFoundException;
@@ -23,12 +24,14 @@ public class ExpenseListService {
     private final ExpenseListRepository repository;
     private final ExpenseRepository expenseRepository;
     private final XpenselyCustomCategoryRepository customCategoryRepository;
+    private final HistoryService historyService;
 
     public ExpenseListService(ExpenseListRepository repository, ExpenseRepository expenseRepository,
-            XpenselyCustomCategoryRepository customCategoryRepository) {
+            XpenselyCustomCategoryRepository customCategoryRepository, HistoryService historyService) {
         this.repository = repository;
         this.expenseRepository = expenseRepository;
         this.customCategoryRepository = customCategoryRepository;
+        this.historyService = historyService;
     }
 
     public ExpenseList createList(ExpenseList list) {
@@ -36,6 +39,7 @@ public class ExpenseListService {
     }
 
     public void deleteById(Long id) {
+        historyService.deleteForList(id);
         repository.deleteById(id);
     }
 
@@ -62,15 +66,17 @@ public class ExpenseListService {
         return repository.findByOwnerUsernameOrSharedWithUsername(username);
     }
 
-    public Expense addExpenseToList(Long expenseListId, Expense expense) {
+    public Expense addExpenseToList(Long expenseListId, Expense expense, AppUser actor) {
         ExpenseList expenseList = repository.findById(expenseListId)
                 .orElseThrow(() -> new ResourceNotFoundException("ExpenseList not found with id: " + expenseListId));
+        expense.setLastModified(LocalDateTime.now());
         expenseList.addExpense(expense);
         repository.save(expenseList);
+        historyService.recordCreated(expenseList, expense, actor);
         return expense;
     }
 
-    public void deleteExpenseFromList(Long expenseListId, Long expenseId) {
+    public void deleteExpenseFromList(Long expenseListId, Long expenseId, AppUser actor) {
         ExpenseList expenseList = repository.findById(expenseListId)
                 .orElseThrow(() -> new ResourceNotFoundException("ExpenseList not found with id: " + expenseListId));
         Expense expenseToRemove = null;
@@ -80,11 +86,11 @@ public class ExpenseListService {
                 break;
             }
         }
-        if (expenseToRemove != null) {
+        if (expenseToRemove == null) {
-            expenseList.removeExpense(expenseToRemove);
-        } else {
             throw new ResourceNotFoundException("Expense not found with id: " + expenseId);
         }
+        historyService.recordDeleted(expenseList, expenseToRemove, actor);
+        expenseList.removeExpense(expenseToRemove);
         repository.save(expenseList);
     }
 
@@ -110,7 +116,7 @@ public class ExpenseListService {
         return repository.findByInviteCode(inviteCode);
     }
 
-    public Expense updateExpense(Long expenseListId, Expense updatedExpense) {
+    public Expense updateExpense(Long expenseListId, Expense updatedExpense, AppUser actor) {
         ExpenseList expenseList = repository.findById(expenseListId)
                 .orElseThrow(() -> new ResourceNotFoundException("ExpenseList not found with id: " + expenseListId));
 
@@ -121,6 +127,9 @@ public class ExpenseListService {
 
         Expense existingExpense = expenseRepository.findById(updatedExpense.getId())
                 .orElseThrow(() -> new ResourceNotFoundException("Expense not found with id: " + updatedExpense.getId()));
+
+        Expense before = snapshot(existingExpense);
+
         existingExpense.setTitle(updatedExpense.getTitle());
         existingExpense.setAmount(updatedExpense.getAmount());
         existingExpense.setPersonalUseAmount(updatedExpense.getPersonalUseAmount());
@@ -128,8 +137,25 @@ public class ExpenseListService {
         existingExpense.setDate(updatedExpense.getDate());
         existingExpense.setOwner(updatedExpense.getOwner());
         existingExpense.setCategory(updatedExpense.getCategory());
+        existingExpense.setLastModified(LocalDateTime.now());
 
-        return expenseRepository.save(existingExpense);
+        Expense saved = expenseRepository.save(existingExpense);
+        historyService.recordUpdated(expenseList, before, saved, actor);
+        return saved;
+    }
+
+    /** Detached copy of the mutable fields, taken before applying an update. */
+    private Expense snapshot(Expense e) {
+        Expense copy = new Expense();
+        copy.setId(e.getId());
+        copy.setTitle(e.getTitle());
+        copy.setAmount(e.getAmount());
+        copy.setPersonalUseAmount(e.getPersonalUseAmount());
+        copy.setOtherPersonAmount(e.getOtherPersonAmount());
+        copy.setCategory(e.getCategory());
+        copy.setDate(e.getDate());
+        copy.setOwner(e.getOwner());
+        return copy;
     }
 
     // TODO implement API for this

src/main/java/de/zendric/app/xpensely_server/services/HistoryService.java

@@ -0,0 +1,125 @@
+package de.zendric.app.xpensely_server.services;
+
+import java.time.LocalDateTime;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+
+import org.springframework.data.domain.Page;
+import org.springframework.data.domain.PageRequest;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+import tools.jackson.core.JacksonException;
+import tools.jackson.core.type.TypeReference;
+import tools.jackson.databind.ObjectMapper;
+
+import de.zendric.app.xpensely_server.model.AppUser;
+import de.zendric.app.xpensely_server.model.DTO.HistoryEntryDto;
+import de.zendric.app.xpensely_server.model.DTO.HistoryPageDto;
+import de.zendric.app.xpensely_server.model.Expense;
+import de.zendric.app.xpensely_server.model.ExpenseHistoryEntry;
+import de.zendric.app.xpensely_server.model.ExpenseList;
+import de.zendric.app.xpensely_server.model.FieldChange;
+import de.zendric.app.xpensely_server.model.HistoryEntryType;
+import de.zendric.app.xpensely_server.repo.ExpenseHistoryRepository;
+
+@Service
+@Transactional
+public class HistoryService {
+
+    private final ExpenseHistoryRepository historyRepository;
+    private final ObjectMapper objectMapper;
+
+    public HistoryService(ExpenseHistoryRepository historyRepository, ObjectMapper objectMapper) {
+        this.historyRepository = historyRepository;
+        this.objectMapper = objectMapper;
+    }
+
+    public void recordCreated(ExpenseList list, Expense expense, AppUser actor) {
+        historyRepository.save(baseEntry(list, expense, actor, HistoryEntryType.CREATED));
+    }
+
+    public void recordUpdated(ExpenseList list, Expense before, Expense after, AppUser actor) {
+        List<FieldChange> changes = diff(before, after);
+        if (changes.isEmpty())
+            return;
+        ExpenseHistoryEntry entry = baseEntry(list, after, actor, HistoryEntryType.UPDATED);
+        try {
+            entry.setChanges(objectMapper.writeValueAsString(changes));
+        } catch (JacksonException e) {
+            throw new IllegalStateException("Failed to serialize history changes", e);
+        }
+        historyRepository.save(entry);
+    }
+
+    public void recordDeleted(ExpenseList list, Expense expense, AppUser actor) {
+        historyRepository.save(baseEntry(list, expense, actor, HistoryEntryType.DELETED));
+    }
+
+    public void deleteForList(Long listId) {
+        historyRepository.deleteByExpenseListId(listId);
+    }
+
+    public List<FieldChange> diff(Expense before, Expense after) {
+        List<FieldChange> changes = new ArrayList<>();
+        addIfChanged(changes, "title", before.getTitle(), after.getTitle());
+        addIfChanged(changes, "amount", before.getAmount(), after.getAmount());
+        addIfChanged(changes, "personalUseAmount", before.getPersonalUseAmount(), after.getPersonalUseAmount());
+        addIfChanged(changes, "otherPersonAmount", before.getOtherPersonAmount(), after.getOtherPersonAmount());
+        addIfChanged(changes, "category", before.getCategory(), after.getCategory());
+        addIfChanged(changes, "date", before.getDate(), after.getDate());
+        addIfChanged(changes, "owner",
+                before.getOwner() == null ? null : before.getOwner().getUsername(),
+                after.getOwner() == null ? null : after.getOwner().getUsername());
+        return changes;
+    }
+
+    private ExpenseHistoryEntry baseEntry(ExpenseList list, Expense expense, AppUser actor, HistoryEntryType type) {
+        ExpenseHistoryEntry entry = new ExpenseHistoryEntry();
+        entry.setExpenseList(list);
+        entry.setExpenseId(expense.getId());
+        entry.setExpenseTitle(expense.getTitle());
+        entry.setActor(actor);
+        entry.setType(type);
+        entry.setTimestamp(LocalDateTime.now());
+        return entry;
+    }
+
+    private void addIfChanged(List<FieldChange> changes, String field, Object oldVal, Object newVal) {
+        if (!Objects.equals(oldVal, newVal)) {
+            changes.add(new FieldChange(field,
+                    oldVal == null ? null : String.valueOf(oldVal),
+                    newVal == null ? null : String.valueOf(newVal)));
+        }
+    }
+
+    @Transactional(readOnly = true)
+    public HistoryPageDto getHistory(Long listId, int page, int size) {
+        int cappedSize = Math.min(Math.max(size, 1), 100);
+        Page<ExpenseHistoryEntry> result = historyRepository
+                .findByExpenseListIdOrderByTimestampDescIdDesc(listId,
+                        PageRequest.of(Math.max(page, 0), cappedSize));
+        List<HistoryEntryDto> entries = result.getContent().stream().map(this::toDto).toList();
+        return new HistoryPageDto(entries, result.hasNext());
+    }
+
+    private HistoryEntryDto toDto(ExpenseHistoryEntry entry) {
+        List<FieldChange> changes = List.of();
+        if (entry.getChanges() != null) {
+            try {
+                changes = objectMapper.readValue(entry.getChanges(),
+                        new TypeReference<List<FieldChange>>() {
+                        });
+            } catch (JacksonException e) {
+                throw new IllegalStateException("Corrupt history changes for entry " + entry.getId(), e);
+            }
+        }
+        return new HistoryEntryDto(
+                entry.getType().name(),
+                entry.getActor() == null ? null : entry.getActor().getUsername(),
+                entry.getExpenseTitle(),
+                entry.getTimestamp(),
+                changes);
+    }
+}

src/test/java/de/zendric/app/xpensely_Server/controller/ExpenseListControllerTest.java

@@ -2,11 +2,13 @@ package de.zendric.app.xpensely_Server.controller;
 
 import de.zendric.app.xpensely_server.controller.ExpenseListController;
 import de.zendric.app.xpensely_server.model.AppUser;
+import de.zendric.app.xpensely_server.model.DTO.HistoryPageDto;
 import de.zendric.app.xpensely_server.model.Expense;
 import de.zendric.app.xpensely_server.model.ExpenseList;
 import de.zendric.app.xpensely_server.security.AuthenticatedUserResolver;
 import de.zendric.app.xpensely_server.services.CategoryService;
 import de.zendric.app.xpensely_server.services.ExpenseListService;
+import de.zendric.app.xpensely_server.services.HistoryService;
 import org.junit.jupiter.api.Test;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.security.oauth2.server.resource.autoconfigure.servlet.OAuth2ResourceServerAutoConfiguration;
@@ -38,6 +40,7 @@ class ExpenseListControllerTest {
     @MockitoBean ExpenseListService expenseListService;
     @MockitoBean CategoryService categoryService;
     @MockitoBean AuthenticatedUserResolver authenticatedUserResolver;
+    @MockitoBean HistoryService historyService;
 
     // --- Validation tests ---
 
@@ -199,7 +202,7 @@ class ExpenseListControllerTest {
 
         when(expenseListService.findById(1L)).thenReturn(Optional.of(list));
         when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(member);
-        when(expenseListService.addExpenseToList(eq(1L), any())).thenReturn(new Expense());
+        when(expenseListService.addExpenseToList(eq(1L), any(), any())).thenReturn(new Expense());
 
         mockMvc.perform(post("/api/expenselist/1/add")
                 .contentType(MediaType.APPLICATION_JSON)
@@ -281,4 +284,42 @@ class ExpenseListControllerTest {
                 .content("{\"inviteCode\":\"ABC123\"}"))
                 .andExpect(status().isConflict());
     }
+
+    // --- History endpoint ---
+
+    @Test
+    void getHistory_member_returns200WithPage() throws Exception {
+        AppUser owner = new AppUser(); owner.setId(1L); owner.setUsername("alice");
+        ExpenseList list = new ExpenseList(); list.setId(1L); list.setOwner(owner);
+        when(expenseListService.findById(1L)).thenReturn(Optional.of(list));
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(owner);
+        when(historyService.getHistory(1L, 0, 30))
+                .thenReturn(new HistoryPageDto(List.of(), false));
+
+        mockMvc.perform(get("/api/expenselist/1/history"))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$.entries").isArray())
+                .andExpect(jsonPath("$.hasMore").value(false));
+    }
+
+    @Test
+    void getHistory_nonMember_returns403() throws Exception {
+        AppUser owner = new AppUser(); owner.setId(1L);
+        AppUser stranger = new AppUser(); stranger.setId(2L);
+        ExpenseList list = new ExpenseList(); list.setId(1L); list.setOwner(owner);
+        when(expenseListService.findById(1L)).thenReturn(Optional.of(list));
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(stranger);
+
+        mockMvc.perform(get("/api/expenselist/1/history"))
+                .andExpect(status().isForbidden());
+    }
+
+    @Test
+    void getHistory_unknownList_returns404() throws Exception {
+        when(expenseListService.findById(99L)).thenReturn(Optional.empty());
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(new AppUser());
+
+        mockMvc.perform(get("/api/expenselist/99/history"))
+                .andExpect(status().isNotFound());
+    }
 }

src/test/java/de/zendric/app/xpensely_Server/preparation/HistoryBackfillRunnerTest.java

@@ -0,0 +1,92 @@
+package de.zendric.app.xpensely_Server.preparation;
+
+import java.time.LocalDate;
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.ArgumentCaptor;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+import de.zendric.app.xpensely_server.model.AppUser;
+import de.zendric.app.xpensely_server.model.Expense;
+import de.zendric.app.xpensely_server.model.ExpenseHistoryEntry;
+import de.zendric.app.xpensely_server.model.ExpenseList;
+import de.zendric.app.xpensely_server.model.HistoryEntryType;
+import de.zendric.app.xpensely_server.preparation.HistoryBackfillRunner;
+import de.zendric.app.xpensely_server.repo.ExpenseHistoryRepository;
+import de.zendric.app.xpensely_server.repo.ExpenseRepository;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+@ExtendWith(MockitoExtension.class)
+class HistoryBackfillRunnerTest {
+
+    @Mock ExpenseRepository expenseRepository;
+    @Mock ExpenseHistoryRepository historyRepository;
+
+    @InjectMocks
+    HistoryBackfillRunner runner;
+
+    private Expense expense(long id, LocalDate date) {
+        AppUser owner = new AppUser(); owner.setId(1L); owner.setUsername("alice");
+        ExpenseList list = new ExpenseList(); list.setId(10L);
+        Expense e = new Expense();
+        e.setId(id);
+        e.setTitle("Groceries");
+        e.setDate(date);
+        e.setOwner(owner);
+        e.setExpenseList(list);
+        return e;
+    }
+
+    @Test
+    void fillsLastModifiedFromDate_forExpensesMissingIt() throws Exception {
+        Expense e = expense(42L, LocalDate.of(2026, 6, 1));
+        when(expenseRepository.findByLastModifiedIsNull()).thenReturn(List.of(e));
+        when(expenseRepository.findAll()).thenReturn(List.of(e));
+        when(historyRepository.findAllExpenseIds()).thenReturn(List.of(42L));
+
+        runner.run();
+
+        assertThat(e.getLastModified()).isEqualTo(LocalDate.of(2026, 6, 1).atStartOfDay());
+        verify(expenseRepository).save(e);
+    }
+
+    @Test
+    void createsSyntheticCreatedEntry_forExpensesWithoutHistory() throws Exception {
+        Expense e = expense(42L, LocalDate.of(2026, 6, 1));
+        when(expenseRepository.findByLastModifiedIsNull()).thenReturn(List.of());
+        when(expenseRepository.findAll()).thenReturn(List.of(e));
+        when(historyRepository.findAllExpenseIds()).thenReturn(List.of());
+
+        runner.run();
+
+        ArgumentCaptor<ExpenseHistoryEntry> captor = ArgumentCaptor.forClass(ExpenseHistoryEntry.class);
+        verify(historyRepository).save(captor.capture());
+        ExpenseHistoryEntry entry = captor.getValue();
+        assertThat(entry.getType()).isEqualTo(HistoryEntryType.CREATED);
+        assertThat(entry.getExpenseId()).isEqualTo(42L);
+        assertThat(entry.getExpenseTitle()).isEqualTo("Groceries");
+        assertThat(entry.getActor().getUsername()).isEqualTo("alice");
+        assertThat(entry.getTimestamp()).isEqualTo(LocalDate.of(2026, 6, 1).atStartOfDay());
+    }
+
+    @Test
+    void isIdempotent_skipsExpensesThatAlreadyHaveHistory() throws Exception {
+        Expense e = expense(42L, LocalDate.of(2026, 6, 1));
+        when(expenseRepository.findByLastModifiedIsNull()).thenReturn(List.of());
+        when(expenseRepository.findAll()).thenReturn(List.of(e));
+        when(historyRepository.findAllExpenseIds()).thenReturn(List.of(42L));
+
+        runner.run();
+
+        verify(historyRepository, never()).save(any());
+    }
+}

src/test/java/de/zendric/app/xpensely_Server/services/ExpenseListServiceTest.java

@@ -1,20 +1,27 @@
 package de.zendric.app.xpensely_Server.services;
 
 import de.zendric.app.xpensely_server.model.AppUser;
+import de.zendric.app.xpensely_server.model.Expense;
 import de.zendric.app.xpensely_server.model.ExpenseList;
 import de.zendric.app.xpensely_server.repo.ExpenseListRepository;
 import de.zendric.app.xpensely_server.repo.ExpenseRepository;
 import de.zendric.app.xpensely_server.repo.XpenselyCustomCategoryRepository;
 import de.zendric.app.xpensely_server.services.ExpenseListService;
+import de.zendric.app.xpensely_server.services.HistoryService;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
 import org.mockito.InjectMocks;
 import org.mockito.Mock;
 import org.mockito.junit.jupiter.MockitoExtension;
 
+import java.time.LocalDate;
+import java.util.ArrayList;
 import java.util.List;
+import java.util.Optional;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.any;
+import static org.mockito.Mockito.eq;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 import static org.mockito.Mockito.never;
@@ -25,6 +32,7 @@ class ExpenseListServiceTest {
     @Mock ExpenseListRepository repository;
     @Mock ExpenseRepository expenseRepository;
     @Mock XpenselyCustomCategoryRepository customCategoryRepository;
+    @Mock HistoryService historyService;
 
     @InjectMocks
     ExpenseListService service;
@@ -54,4 +62,72 @@ class ExpenseListServiceTest {
         verify(repository).findByOwnerUsernameOrSharedWithUsername("alice");
         verify(repository, never()).findAll();
     }
+
+    private Expense makeExpense(Long id, String title, double amount) {
+        Expense e = new Expense();
+        e.setId(id);
+        e.setTitle(title);
+        e.setAmount(amount);
+        e.setPersonalUseAmount(0.0);
+        e.setOtherPersonAmount(0.0);
+        e.setDate(LocalDate.of(2026, 7, 1));
+        return e;
+    }
+
+    @Test
+    void addExpenseToList_setsLastModified_andRecordsCreated() {
+        AppUser actor = new AppUser(); actor.setId(1L);
+        ExpenseList list = new ExpenseList(); list.setId(10L);
+        list.setExpenses(new ArrayList<>());
+        when(repository.findById(10L)).thenReturn(Optional.of(list));
+
+        Expense expense = makeExpense(null, "Lunch", 10.0);
+        service.addExpenseToList(10L, expense, actor);
+
+        assertThat(expense.getLastModified()).isNotNull();
+        verify(historyService).recordCreated(list, expense, actor);
+    }
+
+    @Test
+    void updateExpense_setsLastModified_andRecordsUpdatedWithBeforeSnapshot() {
+        AppUser actor = new AppUser(); actor.setId(1L);
+        ExpenseList list = new ExpenseList(); list.setId(10L);
+        Expense existing = makeExpense(42L, "Old title", 12.0);
+        list.setExpenses(new ArrayList<>(List.of(existing)));
+        when(repository.findById(10L)).thenReturn(Optional.of(list));
+        when(expenseRepository.findById(42L)).thenReturn(Optional.of(existing));
+        when(expenseRepository.save(any(Expense.class))).thenAnswer(inv -> inv.getArgument(0));
+
+        Expense updated = makeExpense(42L, "New title", 15.0);
+        service.updateExpense(10L, updated, actor);
+
+        assertThat(existing.getLastModified()).isNotNull();
+        org.mockito.ArgumentCaptor<Expense> beforeCaptor =
+                org.mockito.ArgumentCaptor.forClass(Expense.class);
+        verify(historyService).recordUpdated(eq(list), beforeCaptor.capture(), eq(existing), eq(actor));
+        assertThat(beforeCaptor.getValue().getTitle()).isEqualTo("Old title");
+        assertThat(beforeCaptor.getValue().getAmount()).isEqualTo(12.0);
+    }
+
+    @Test
+    void deleteById_deletesHistoryEntriesBeforeDeletingList() {
+        service.deleteById(10L);
+
+        org.mockito.InOrder inOrder = org.mockito.Mockito.inOrder(historyService, repository);
+        inOrder.verify(historyService).deleteForList(10L);
+        inOrder.verify(repository).deleteById(10L);
+    }
+
+    @Test
+    void deleteExpenseFromList_recordsDeleted() {
+        AppUser actor = new AppUser(); actor.setId(1L);
+        ExpenseList list = new ExpenseList(); list.setId(10L);
+        Expense existing = makeExpense(42L, "Taxi", 9.0);
+        list.setExpenses(new ArrayList<>(List.of(existing)));
+        when(repository.findById(10L)).thenReturn(Optional.of(list));
+
+        service.deleteExpenseFromList(10L, 42L, actor);
+
+        verify(historyService).recordDeleted(eq(list), any(Expense.class), eq(actor));
+    }
 }

src/test/java/de/zendric/app/xpensely_Server/services/HistoryServiceTest.java

@@ -0,0 +1,209 @@
+package de.zendric.app.xpensely_Server.services;
+
+import java.time.LocalDate;
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.ArgumentCaptor;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.Spy;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.data.domain.PageImpl;
+import org.springframework.data.domain.PageRequest;
+import org.springframework.data.domain.Pageable;
+
+import tools.jackson.databind.ObjectMapper;
+
+import de.zendric.app.xpensely_server.model.AppUser;
+import de.zendric.app.xpensely_server.model.DTO.HistoryPageDto;
+import de.zendric.app.xpensely_server.model.Expense;
+import de.zendric.app.xpensely_server.model.ExpenseHistoryEntry;
+import de.zendric.app.xpensely_server.model.ExpenseList;
+import de.zendric.app.xpensely_server.model.FieldChange;
+import de.zendric.app.xpensely_server.model.HistoryEntryType;
+import de.zendric.app.xpensely_server.repo.ExpenseHistoryRepository;
+import de.zendric.app.xpensely_server.services.HistoryService;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+@ExtendWith(MockitoExtension.class)
+class HistoryServiceTest {
+
+    @Mock ExpenseHistoryRepository historyRepository;
+    @Spy ObjectMapper objectMapper = new ObjectMapper();
+
+    @InjectMocks
+    HistoryService service;
+
+    private AppUser user(long id, String name) {
+        AppUser u = new AppUser();
+        u.setId(id);
+        u.setUsername(name);
+        return u;
+    }
+
+    private Expense expense(String title, double amount, String category, LocalDate date, AppUser owner) {
+        Expense e = new Expense();
+        e.setId(42L);
+        e.setTitle(title);
+        e.setAmount(amount);
+        e.setPersonalUseAmount(0.0);
+        e.setOtherPersonAmount(0.0);
+        e.setCategory(category);
+        e.setDate(date);
+        e.setOwner(owner);
+        return e;
+    }
+
+    @Test
+    void diff_detectsChangedFields_withOldAndNewValues() {
+        AppUser alice = user(1L, "alice");
+        Expense before = expense("Groceries", 12.0, "Food", LocalDate.of(2026, 7, 1), alice);
+        Expense after = expense("Groceries", 15.5, "Household", LocalDate.of(2026, 7, 1), alice);
+
+        List<FieldChange> changes = service.diff(before, after);
+
+        assertThat(changes).containsExactlyInAnyOrder(
+                new FieldChange("amount", "12.0", "15.5"),
+                new FieldChange("category", "Food", "Household"));
+    }
+
+    @Test
+    void diff_ownerChange_isReportedByUsername() {
+        Expense before = expense("Taxi", 9.0, null, LocalDate.of(2026, 7, 1), user(1L, "alice"));
+        Expense after = expense("Taxi", 9.0, null, LocalDate.of(2026, 7, 1), user(2L, "ben"));
+
+        List<FieldChange> changes = service.diff(before, after);
+
+        assertThat(changes).containsExactly(new FieldChange("owner", "alice", "ben"));
+    }
+
+    @Test
+    void diff_nullToValue_isReported() {
+        Expense before = expense("Taxi", 9.0, null, LocalDate.of(2026, 7, 1), user(1L, "alice"));
+        Expense after = expense("Taxi", 9.0, "Transportation", LocalDate.of(2026, 7, 1), user(1L, "alice"));
+
+        assertThat(service.diff(before, after))
+                .containsExactly(new FieldChange("category", null, "Transportation"));
+    }
+
+    @Test
+    void recordUpdated_noChanges_writesNothing() {
+        AppUser alice = user(1L, "alice");
+        Expense same = expense("Groceries", 12.0, "Food", LocalDate.of(2026, 7, 1), alice);
+        ExpenseList list = new ExpenseList();
+        list.setId(5L);
+
+        service.recordUpdated(list, same, same, alice);
+
+        verify(historyRepository, never()).save(any());
+    }
+
+    @Test
+    void recordUpdated_savesEntryWithJsonChanges() {
+        AppUser alice = user(1L, "alice");
+        ExpenseList list = new ExpenseList();
+        list.setId(5L);
+        Expense before = expense("Groceries", 12.0, "Food", LocalDate.of(2026, 7, 1), alice);
+        Expense after = expense("Groceries", 15.5, "Food", LocalDate.of(2026, 7, 1), alice);
+
+        service.recordUpdated(list, before, after, alice);
+
+        ArgumentCaptor<ExpenseHistoryEntry> captor = ArgumentCaptor.forClass(ExpenseHistoryEntry.class);
+        verify(historyRepository).save(captor.capture());
+        ExpenseHistoryEntry entry = captor.getValue();
+        assertThat(entry.getType()).isEqualTo(HistoryEntryType.UPDATED);
+        assertThat(entry.getExpenseId()).isEqualTo(42L);
+        assertThat(entry.getExpenseTitle()).isEqualTo("Groceries");
+        assertThat(entry.getActor()).isEqualTo(alice);
+        assertThat(entry.getExpenseList()).isEqualTo(list);
+        assertThat(entry.getTimestamp()).isNotNull();
+        assertThat(entry.getChanges())
+                .isEqualTo("[{\"field\":\"amount\",\"old\":\"12.0\",\"new\":\"15.5\"}]");
+    }
+
+    @Test
+    void recordCreated_savesEntryWithoutChanges() {
+        AppUser alice = user(1L, "alice");
+        ExpenseList list = new ExpenseList();
+        list.setId(5L);
+        Expense e = expense("Groceries", 12.0, "Food", LocalDate.of(2026, 7, 1), alice);
+
+        service.recordCreated(list, e, alice);
+
+        ArgumentCaptor<ExpenseHistoryEntry> captor = ArgumentCaptor.forClass(ExpenseHistoryEntry.class);
+        verify(historyRepository).save(captor.capture());
+        assertThat(captor.getValue().getType()).isEqualTo(HistoryEntryType.CREATED);
+        assertThat(captor.getValue().getChanges()).isNull();
+    }
+
+    @Test
+    void recordDeleted_savesEntry() {
+        AppUser alice = user(1L, "alice");
+        ExpenseList list = new ExpenseList();
+        list.setId(5L);
+        Expense e = expense("Taxi", 9.0, null, LocalDate.of(2026, 7, 1), alice);
+
+        service.recordDeleted(list, e, alice);
+
+        ArgumentCaptor<ExpenseHistoryEntry> captor = ArgumentCaptor.forClass(ExpenseHistoryEntry.class);
+        verify(historyRepository).save(captor.capture());
+        assertThat(captor.getValue().getType()).isEqualTo(HistoryEntryType.DELETED);
+        assertThat(captor.getValue().getExpenseTitle()).isEqualTo("Taxi");
+    }
+
+    @Test
+    void getHistory_mapsEntriesToDto_andParsesChangesJson() {
+        ExpenseHistoryEntry entry = new ExpenseHistoryEntry();
+        entry.setType(HistoryEntryType.UPDATED);
+        entry.setActor(user(1L, "alice"));
+        entry.setExpenseTitle("Groceries");
+        entry.setTimestamp(java.time.LocalDateTime.of(2026, 7, 6, 14, 31));
+        entry.setChanges("[{\"field\":\"amount\",\"old\":\"12.0\",\"new\":\"15.5\"}]");
+        when(historyRepository.findByExpenseListIdOrderByTimestampDescIdDesc(eq(5L), any(Pageable.class)))
+                .thenReturn(new PageImpl<>(List.of(entry), PageRequest.of(0, 30), 31));
+
+        HistoryPageDto page = service.getHistory(5L, 0, 30);
+
+        assertThat(page.hasMore()).isTrue();
+        assertThat(page.entries()).hasSize(1);
+        assertThat(page.entries().get(0).type()).isEqualTo("UPDATED");
+        assertThat(page.entries().get(0).actorUsername()).isEqualTo("alice");
+        assertThat(page.entries().get(0).changes())
+                .containsExactly(new FieldChange("amount", "12.0", "15.5"));
+    }
+
+    @Test
+    void getHistory_capsPageSizeAt100() {
+        when(historyRepository.findByExpenseListIdOrderByTimestampDescIdDesc(eq(5L), any(Pageable.class)))
+                .thenReturn(new PageImpl<>(List.of()));
+
+        service.getHistory(5L, 0, 5000);
+
+        ArgumentCaptor<Pageable> captor = ArgumentCaptor.forClass(Pageable.class);
+        verify(historyRepository).findByExpenseListIdOrderByTimestampDescIdDesc(eq(5L), captor.capture());
+        assertThat(captor.getValue().getPageSize()).isEqualTo(100);
+    }
+
+    @Test
+    void getHistory_nullChanges_yieldsEmptyList() {
+        ExpenseHistoryEntry entry = new ExpenseHistoryEntry();
+        entry.setType(HistoryEntryType.CREATED);
+        entry.setActor(user(1L, "alice"));
+        entry.setExpenseTitle("Groceries");
+        entry.setTimestamp(java.time.LocalDateTime.of(2026, 7, 6, 14, 31));
+        when(historyRepository.findByExpenseListIdOrderByTimestampDescIdDesc(eq(5L), any(Pageable.class)))
+                .thenReturn(new PageImpl<>(List.of(entry)));
+
+        HistoryPageDto page = service.getHistory(5L, 0, 30);
+
+        assertThat(page.entries().get(0).changes()).isEmpty();
+    }
+}