如何为实体对象加载导航集合的属性

Question

我正在处理一个以.net 5 和Entity framework core 为目标的Asp,net core 5 项目。

我有这些实体：

学生：

[ Key ]                                          public string      Id                         { get; set; }
[ Required ]                                     public string      FirstName                  { get; set; }
[ Required ]                                     public string      FamilyName                 { get; set; }
[ Required ]                                     public DateTime    BirthDate                  { get; set; }
[ Required ]                                     public string      BirthPlace                 { get; set; }
[ Required ]                                     public Gender      Gender                     { get; set; }
[ Required ]                                     public Nationality Nationality                { get; set; }
[ Required ]                                     public string      Address                    { get; set; }
[ Required ]                                     public DateTime    DateOfRegistration         { get; set; }
[ Required ]                                     public int         StudyLevelId               { get; set; }
[ Required ] [ForeignKey( nameof(AcademicYear))] public string      AcademicYearOfRegistration { get; set; }
[ Required ]                                     public string      GroupId                    { get; set; }
[ Phone ]                                        public string      PhoneNumber                { get; set; }
[ EmailAddress ]                                 public string      Email                      { get; set; }

public string FullName          => $"{FirstName} {FamilyName}";
public string NationalId        { get; set; }
public string PassportId        { get; set; }
public string ResideneCardId    { get; set; }
public string StudentGuardianId { get; set; }
public byte[] Picture           { get; set; }


public virtual StudentGuadian                      StudentGuadian   { get; set; }
public virtual StudyLevel                          StudyLevel       { get; set; }
public virtual AcademicYear                        AcademicYear     { get; set; }
public virtual Group                               Group            { get; set; }
public virtual ICollection<StudentAbsence>         Absences         { get; set; }
public virtual ICollection<TestMark>               TestMarks        { get; set; }
public virtual ICollection<ExamMark>               ExamMarks        { get; set; }
public virtual ICollection<StudentPayment>         Payements        { get; set; }
public virtual ICollection<StudentMonthlyPayement> MonthlyPayements { get; set; }
public virtual StudentTrainingMark                 TrainingMark     { get; set; }
public virtual StudentProjectMark                  ProjectMark      { get; set; }
public virtual ICollection<StudentYearlyResult>    YearlyResults    { get; set; }
public virtual StudentDiplomaResult                DiplomaResult    { get; set; }

学生缺席：

 [ Key ] [ DatabaseGenerated( DatabaseGeneratedOption.Identity ) ] public int      Id              { get; set; }
        [ Required ]                                                      public DateTime AbsenceDate     { get; set; }
        [ Required ]                                                      public TimeSpan AbsenceTime     { get; set; }
        [ Required ]                                                      public bool     IsJustified     { get; set; }
        [ Required ]                                                      public string   StudentId       { get; set; }
        [ Required ]                                                      public string   SchoolSubjectId { get; set; }

        public byte[] DocOfJustify { get; set; }
        public string Note         { get; set; }
        public string Reason       { get; set; }


        public virtual Student       Student       { get; set; }
        public virtual SchoolSubject SchoolSubject { get; set; }

        public string   CreatedBy    { get; set; }
        public DateTime CreatedOn    { get; set; }
        public bool     IsEdited     { get; set; }
        public string   LastEditor   { get; set; }
        public DateTime LastEditDate { get; set; }

学科：

        [Key] 
        public string Id { get; set; }
        
        [Required]
        public string FullTitle { get; set; }

        [Required]
        public string Coefficient { get; set; }

        [Required]
        public byte NumberOfTests { get; set; }

        [Required]
        public SchoolSubjectStatus Status { get; set; }

        [Required]
        public string BrancheId { get; set; }

        [Required]
        public string SemesterId { get; set; }

        [Required]
        public byte FormationYearId { get; set; }


        public virtual Branche                          Branche              { get; set; }
        public virtual Semester                         Semester             { get; set; }
        public virtual FormationYear                    FormationYear        { get; set; }
        public virtual ICollection<PedagogicalSequence> PedagogicalSequences { get; set; }
        public virtual ICollection<StudentAbsence>      StudentAbsences      { get; set; }
        public virtual ICollection<Test>                Tests                { get; set; }
        public virtual ICollection<Exam>                Exams                { get; set; }
        public virtual ICollection<GroupProfSubject>    Professors           { get; set; }
        public virtual ICollection<ProfAbsence>         ProfAbsences         { get; set; }

形成年：

        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public byte Id { get; set; }

        [Required]
        public string Title { get; set; }

        public virtual ICollection<SchoolSubject> SchoolSubjects { get; set; }

有什么问题？

在一个方法中，我得到一个student 对象，我加载了他所有的Absences，我还尝试加载Absence.SchoolSubject 属性（来自Absence）和Absence.SchoolSubject.FormationYear 属性（来自Absence） 但我收到一条错误消息，他总结说 Absence.SchoolSubject 和 Absence.SchoolSubject.FormationYear 属性是 Null 并且未加载。

我尝试了什么：

                                      Context.ChangeTracker.QueryTrackingBehavior = QueryTrackingBehavior.TrackAll;

                                      IQueryable<Student> query = Context.Set<Student>().Where( s => s.Id == pkValue );

                                      query.Include( s => s.Group ).Include( s => s.Group.Branche )
                                           .Include( s => s.Group.FormationYear )
                                           .Include( s => s.StudentGuadian ).Include( s => s.Absences )
                                           .Include( s => s.Absences.First().SchoolSubject )
                                           .Include( s => s.Absences.First().SchoolSubject.FormationYear )
                                           .Load();

                                      return query.FirstOrDefault();

Context 是 DbContext 对象。

我知道这不是为 Student 对象加载 Absence 嵌套导航属性的正确方法，请帮助解决此问题？

Answer 1

IQueryable 实现是不可变的，因此，您必须存储新的查询。 使用ThenInclude 进行集合导航。我还删除了不需要的包括：

query = query
    .Include( s => s.Group.Branche )
    .Include( s => s.Group.FormationYear )
    .Include( s => s.StudentGuadian )
    .Include( s => s.Absences )
        .ThenInclude( s => s.SchoolSubject.FormationYear );

return query.FirstOrDefault();