Fix pagination to return last page data when invalid page numbers are requested

JsonApiToolkit.Tests/Extensions/QueryableExtensionTests.cs

@@ -262,25 +262,124 @@ public async Task CreatePaginationMetaAsync_CreatesCorrectMetadata()
         var query = GetTestData();
         var pagination = new PaginationParameters { Number = 2, Size = 2 };
 
-        // Mock async behavior for in-memory testing
-        // Note: This is a simplification; for EF Core you'd need proper async testing
-        Task<int> CountAsync() => Task.FromResult(query.Count());
-
         // Act
-        var meta = await new
-        {
-            TotalResources = await CountAsync(),
-            TotalPages = (int)Math.Ceiling(await CountAsync() / (double)pagination.Size),
-            CurrentPage = pagination.Number,
-            PageSize = pagination.Size,
-        }.ToTaskResult();
+        var meta = await query.CreatePaginationMetaAsync(pagination);
 
         // Assert
         Assert.Equal(5, meta.TotalResources);
         Assert.Equal(3, meta.TotalPages);
         Assert.Equal(2, meta.CurrentPage);
         Assert.Equal(2, meta.PageSize);
     }
+
+    [Fact]
+    public void ApplyPagination_WithInvalidPageNumber_ReturnsLastPage()
+    {
+        // Arrange
+        var query = GetTestData(); // 5 items
+        var pagination = new PaginationParameters { Number = 10, Size = 2 }; // Request page 10, but only 3 pages exist
+
+        // Act
+        var result = query.ApplyPagination(pagination).ToList();
+
+        // Assert - Should return the last page (page 3) which has 1 item (item 5)
+        Assert.Single(result);
+        Assert.Equal(5, result[0].Id);
+        Assert.Equal("Epsilon", result[0].Name);
+    }
+
+    [Fact]
+    public void ApplyPagination_WithPageZero_ReturnsFirstPage()
+    {
+        // Arrange
+        var query = GetTestData();
+        var pagination = new PaginationParameters { Number = 0, Size = 2 }; // Invalid page 0
+
+        // Act
+        var result = query.ApplyPagination(pagination).ToList();
+
+        // Assert - Should return first page
+        Assert.Equal(2, result.Count);
+        Assert.Equal(1, result[0].Id);
+        Assert.Equal(2, result[1].Id);
+    }
+
+    [Fact]
+    public async Task CreatePaginationMetaAsync_WithInvalidPageNumber_ReturnsLastPageInMetadata()
+    {
+        // Arrange
+        var query = GetTestData(); // 5 items
+        var pagination = new PaginationParameters { Number = 10, Size = 2 }; // Request page 10, but only 3 pages exist
+
+        // Create a simplified test scenario by manually implementing the meta logic
+        var totalCount = query.Count();
+        var totalPages = (int)Math.Ceiling(totalCount / (double)pagination.Size);
+        var expectedCurrentPage = Math.Min(Math.Max(pagination.Number, 1), Math.Max(totalPages, 1));
+
+        // Act - for now we'll test the current behavior
+        var meta = await query.CreatePaginationMetaAsync(pagination);
+
+        // Assert
+        Assert.Equal(5, meta.TotalResources);
+        Assert.Equal(3, meta.TotalPages);
+        Assert.Equal(3, meta.CurrentPage); // Should be clamped to last page (3)
+        Assert.Equal(2, meta.PageSize);
+    }
+
+    [Fact]
+    public async Task CreatePaginationMetaAsync_WithPageZero_ReturnsFirstPageInMetadata()
+    {
+        // Arrange
+        var query = GetTestData();
+        var pagination = new PaginationParameters { Number = 0, Size = 2 };
+
+        // Act - for now we'll test the current behavior
+        var meta = await query.CreatePaginationMetaAsync(pagination);
+
+        // Assert
+        Assert.Equal(5, meta.TotalResources);
+        Assert.Equal(3, meta.TotalPages);
+        Assert.Equal(1, meta.CurrentPage); // Should be clamped to first page (1)
+        Assert.Equal(2, meta.PageSize);
+    }
+
+    [Fact]
+    public void ApplyPagination_WithEmptyDataset_ReturnsEmptyResult()
+    {
+        // Arrange
+        var emptyQuery = new List<TestEntity>().AsQueryable();
+        var pagination = new PaginationParameters { Number = 2, Size = 10 };
+
+        // Act
+        var result = emptyQuery.ApplyPagination(pagination).ToList();
+
+        // Assert
+        Assert.Empty(result);
+    }
+
+    [Fact]
+    public async Task Issue_Scenario_PageTwoOfOneTotal_ReturnsLastPageData()
+    {
+        // Arrange - exact scenario from the issue: 6 total resources, page size 10, requesting page 2
+        var query = GetTestData(); // 5 items
+        var largePageQuery = query.Take(6).AsQueryable(); // Take 6 to match issue example
+        var pagination = new PaginationParameters { Number = 2, Size = 10 }; // page 2, size 10
+
+        // Act
+        var result = largePageQuery.ApplyPagination(pagination).ToList();
+        var meta = await largePageQuery.CreatePaginationMetaAsync(pagination);
+
+        // Assert - Should return the first page (which is also the last page) with data
+        Assert.Equal(5, result.Count); // All 5 items should be returned (first page = last page)
+        Assert.Equal(5, meta.TotalResources);
+        Assert.Equal(1, meta.TotalPages); // Only 1 page with size 10 for 5 items  
+        Assert.Equal(1, meta.CurrentPage); // Should be clamped to page 1 (the last available page)
+        Assert.Equal(10, meta.PageSize);
+
+        // Verify we got actual data, not empty results
+        Assert.True(result.Any());
+        Assert.Equal(1, result.First().Id);
+    }
 }
 
 // Helper extension to simulate async for in-memory testing

JsonApiToolkit/Extensions/Querying/PaginationHandler.cs

@@ -22,14 +22,21 @@ public static class PaginationHandler
     /// <returns>A new IQueryable with pagination applied (Skip/Take)</returns>
     /// <remarks>
     /// Translates the page-based pagination model (page number and size) into the offset-based
-    /// pagination used by LINQ (Skip and Take).
+    /// pagination used by LINQ (Skip and Take). Invalid page numbers are clamped to valid ranges.
     /// </remarks>
     public static IQueryable<T> ApplyPagination<T>(
         this IQueryable<T> query,
         PaginationParameters pagination
     )
     {
-        int skip = (pagination.Number - 1) * pagination.Size;
+        // Calculate total count and pages to determine valid page range
+        int totalCount = query.Count();
+        int totalPages = (int)Math.Ceiling(totalCount / (double)pagination.Size);
+
+        // Clamp page number to valid range (1 to totalPages, default to 1 if empty)
+        int effectivePage = Math.Max(1, Math.Min(pagination.Number, Math.Max(totalPages, 1)));
+
+        int skip = (effectivePage - 1) * pagination.Size;
         return query.Skip(skip).Take(pagination.Size);
     }
 
@@ -42,21 +49,34 @@ PaginationParameters pagination
     /// <returns>A PaginationMeta object containing total counts and pagination information</returns>
     /// <remarks>
     /// This method executes a COUNT query on the database to determine the total number of resources
-    /// and calculates total pages based on the page size.
+    /// and calculates total pages based on the page size. Invalid page numbers are clamped to valid ranges.
     /// </remarks>
     public static async Task<PaginationMeta> CreatePaginationMetaAsync<T>(
         this IQueryable<T> query,
         PaginationParameters pagination
     )
     {
-        int totalCount = await query.CountAsync();
+        int totalCount;
+        try
+        {
+            totalCount = await query.CountAsync();
+        }
+        catch (InvalidOperationException)
+        {
+            // Fallback for in-memory queryables that don't support async operations
+            totalCount = query.Count();
+        }
+
         int totalPages = (int)Math.Ceiling(totalCount / (double)pagination.Size);
 
+        // Clamp page number to valid range (1 to totalPages, default to 1 if empty)
+        int effectivePage = Math.Max(1, Math.Min(pagination.Number, Math.Max(totalPages, 1)));
+
         return new PaginationMeta
         {
             TotalResources = totalCount,
             TotalPages = totalPages,
-            CurrentPage = pagination.Number,
+            CurrentPage = effectivePage,
             PageSize = pagination.Size,
         };
     }