Ef core 6 尝试使用时抛出错误 500.ThenInclude 方法

Question

I have two tables Employee and Skill which are connected using many to many relationship with the EmployeeSkill association class. The query bellow works properly however: 1] When I call employees method I get the correct amount of Counted Skill referenced objects but they are all initialized as zeros and 2] When I try to add ThenInclude query it throws 500 error.

Thank you very much for your time

Edit: One solution was to add to Startup.cs the following line of code:

services.AddControllers().AddJsonOptions(x => x.JsonSerializerOptions.ReferenceHandler = ReferenceHandler.IgnoreCycles);

Server Exception:

An unhandled exception has occurred while executing the request. System.Text.Json.JsonException: A possible object cycle was detected. This can either be due to a cycle or if the object depth is larger than the maximum allowed depth of 32

Client Exception:

Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer[100]
      Unhandled exception rendering component: Response status code does not indicate success: 500 (Internal Server Error).
System.Net.Http.HttpRequestException: Response status code does not indicate success: 500 (Internal Server Error).
   at System.Net.Http.HttpResponseMessage.EnsureSuccessStatusCode()
   at System.Net.Http.HttpClient.GetStreamAsyncCore(HttpRequestMessage request, CancellationToken cancellationToken)
   at SkillMatrix.Client.Services.EmployeeDataService.GetEmployeesAsync() in C:\Users\Onelity-mefraimidis\Downloads\Onelity related\.NET projects\SkillMatrix\SkillMatrix.Client\Services\EmployeeDataService.cs:line 30
   at SkillMatrix.Client.Features.EmployeeView.EmployeeOverviewPage.OnInitializedAsync() in C:\Users\Onelity-mefraimidis\Downloads\Onelity related\.NET projects\SkillMatrix\SkillMatrix.Client\Features\EmployeeView\EmployeeOverviewPage.cs:line 54
   at Microsoft.AspNetCore.Components.ComponentBase.RunInitAndSetParametersAsync()
   at Microsoft.AspNetCore.Components.RenderTree.Renderer.GetErrorHandledTask(Task taskToHandle, ComponentState owningComponentState)

Context (is added correctly in the sql database):

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    base.OnModelCreating(modelBuilder);

    // Configure a Many-to-Many Relationship using Fluent API   
    modelBuilder.Entity<Employee>()
                .HasMany(e => e.Skills)
                .WithMany(s => s.Employees)
                .UsingEntity<EmployeeSkill>(
        j => j.HasOne(m => m.Skill).WithMany(g => g.EmployeeSkills),
        j => j.HasOne(m => m.Employee).WithMany(g => g.EmployeeSkills),
        je =>
        {
            je.HasKey("EmployeesId", "SkillsId");
            je.HasData(
                new EmployeeSkill { EmployeesId = 1, SkillsId = 1 },
                new EmployeeSkill { EmployeesId = 1, SkillsId = 3 },
                new EmployeeSkill { EmployeesId = 1, SkillsId = 4 },
                new EmployeeSkill { EmployeesId = 1, SkillsId = 5 },
                new EmployeeSkill { EmployeesId = 1, SkillsId = 8 },

                new EmployeeSkill { EmployeesId = 2, SkillsId = 1 },
                new EmployeeSkill { EmployeesId = 2, SkillsId = 2 },
                new EmployeeSkill { EmployeesId = 2, SkillsId = 3 },
                new EmployeeSkill { EmployeesId = 2, SkillsId = 11 },
                new EmployeeSkill { EmployeesId = 2, SkillsId = 15 },

                new EmployeeSkill { EmployeesId = 3, SkillsId = 10 },
                new EmployeeSkill { EmployeesId = 3, SkillsId = 22 },
                new EmployeeSkill { EmployeesId = 3, SkillsId = 23 },
                new EmployeeSkill { EmployeesId = 3, SkillsId = 40 },
                new EmployeeSkill { EmployeesId = 3, SkillsId = 47 }
                );
        }
        );

Classes:

public class EmployeeSkill
{
    public int SkillsId;
    public int EmployeesId;

    public Employee Employee;
    public Skill Skill;
}

public class Employee
{
    public int Id { get; set; }

    [Required]
    [StringLength(50, ErrorMessage = "First name is too long.")]
    public string FirstName { get; set; }

    [Required]
    [StringLength(50, ErrorMessage = "Last name is too long.")]
    public string LastName { get; set; }

    public DateTime DateOfBirth { get; set; }

    [Required]
    [EmailAddress]
    public string Email { get; set; }

    [Required]
    [Phone]
    public string PhoneNumber { get; set; }

    public bool Smoker { get; set; }

    public MaritalStatus MaritalStatus { get; set; }

    public Gender Gender { get; set; }

    [StringLength(1000, ErrorMessage = "Comment length cannot exceed 1000 characters.")]
    public string Comment { get; set; }

    public DateTime? JoinedDate { get; set; }

    public DateTime? LastUpdateDate { get; set; }

    public DateTime? ExitDate { get; set; }

    public int JobCategoryId { get; set; }

    public JobCategory JobCategory { get; set; }

    #region Navigation Properties
    public virtual EmployeeAddress Address { get; set; }

    public virtual ICollection<Skill> Skills { get; set; }
    public virtual ICollection<EmployeeSkill> EmployeeSkills { get; set; }

    #endregion
}

public class Skill
{
    public int Id { get; set; }

    [Required]
    [StringLength(50, ErrorMessage = "Name is too long.")]
    public string Name { get; set; }

    public string Description { get; set; }

    public Level Level { get; set; }

    public int SkillCategoryId { get; set; }

    #region Navigation Properties
    public virtual SkillCategory SkillCategory { get; set; }

    public virtual ICollection<Employee> Employees { get; set; }
    public virtual ICollection<EmployeeSkill> EmployeeSkills { get; set; }

    #endregion
}

Service method:

public async Task<IEnumerable<EmployeeSkillResponse>> GetEmployeeSkillsAsync()
{
    return await JsonSerializer.DeserializeAsync<IEnumerable<EmployeeSkillResponse>>(await _httpClient.GetStreamAsync("api/employeeskills"),
                                    new JsonSerializerOptions() { PropertyNameCaseInsensitive = true });
}

Repository:

public IEnumerable<Employee> GetEmployees()
{
    var query1 = _skillMatrixContext.Employees.Include(employee => employee.EmployeeSkills).ToList();

    // initially was query
    //var query = _skillMatrixContext.Employees;

    // this throws 500 error
    //var query2 = _skillMatrixContext.Employees.Include(employee => employee.EmployeeSkills).ThenInclude(skill=>skill.Skill).ToList();
    
    return query1;
}

Controller method:

[HttpGet]
[ProducesResponseType(StatusCodes.Status200OK, Type = typeof(IEnumerable<EmployeeResponse>))]
[ProducesResponseType(StatusCodes.Status400BadRequest)]
[Produces(MediaTypeNames.Application.Json)]
public ActionResult<IEnumerable<EmployeeResponse>> GetEmployees()
{
    var employeesFromRepo = _employeeRepository.GetEmployees();

    return Ok(_mapper.Map<IEnumerable<EmployeeResponse>>(employeesFromRepo));
}

Answer 1

One solution was to add to Startup.cs the following line of code:

services.AddControllers().AddJsonOptions(x => x.JsonSerializerOptions.ReferenceHandler = ReferenceHandler.IgnoreCycles);

Also it must be noted that AsNoTracking() can be used in the end of the query. Quoting Svyatoslav Danyliv: EF Core fixup navigation properties via ChangeTracker and it may cause circular reference in loaded objects - that's why you have exception. AsNoTracking() disables this fixup and change tracking which may speedup loading records. Usually when right API is written, it uses DTO classes for returning information via network and such problem almost never appear