实体框架插入现有的导航属性实体

Question

我正在使用 Entity Framework 6，但我不断收到以下错误：

违反主键约束“PK_AssignmentType”。无法在对象“dbo.AssignmentType”中插入重复键。

如您所见，我尝试将实体 AssignmentType 的状态更改为 unchanged。当我为实体Assignment 分配AssignmentType 的外键，然后将导航属性AssignmentType 设置为null 时，我仍然得到相同的错误（在Assignments 的foreach 循环中，而不是在Comments foreach循环）。

Entity Framework 在哪里跟踪实体AssignmentType，为什么它仍然认为它是一个新实体而不是现有实体？

实体Reporting和Assignment之间存在多对多关系。

    [Route("")]
    [HttpPost]
    public IHttpActionResult Add(ReportingDTO data)
    {            
        Reporting reporting = new Reporting { ID = Guid.NewGuid(), Date = DateTime.Now };                     

        foreach (Assignment assignment in data.Assignments)
        {
            _db.Entry(assignment.AssignmentType).State = EntityState.Unchanged;

            if (assignment.Reporting.Count > 0)
            {
                _db.Entry(assignment).State = EntityState.Added;
                _db.Entry(assignment).State = EntityState.Modified;
            }

            reporting.Assignment.Add(assignment);
        }

        foreach (Comment comment in data.Comments)
        {
            comment.ID = Guid.NewGuid();
            comment.AssignmentID = comment.Assignment.ID;
            comment.Assignment = null;
            comment.ReportingID= reporting.ID;
        }

        using(var transaction = _db.Database.BeginTransaction())
        {
            _db.Reporting.Add(reporting);
            _db.Comments.AddRange(data.Comments);
            _db.SaveChanges();

            transaction.Commit();
        }

        return Ok(reporting);
    }

Reporting.cs

[Table("Reporting")]
public partial class Reporting
{
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
    public Reporting()
    {
        Assignment= new HashSet<Assignment>();
    }

    [Key]
    public Guid ID{ get; set; }

    [Column(TypeName = "date")]
    public DateTime Date{ get; set; }  

    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<Assignment> Assignment{ get; set; }
}

Assignment.cs

[Table("Assignment")]
public partial class Assignment
{
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
    public Assignment()
    {
        Reporting = new HashSet<Reporting>();
    }

    [Key]
    public Guid OID { get; set; }

    [Required]
    [StringLength(100)]
    public string Name { get; set; }

    public Guid AssignmentTypeID { get; set; }

    [Required]
    [StringLength(100)]
    public string Project { get; set; }

    public bool Completed { get; set; }

    [ForeignKey("AssignmentTypeID")]
    public virtual AssignmentType AssignmentType { get; set; }

    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<Reporting> Reporting { get; set; }
}

AssignmentType

[Table("AssignmentType")]
public partial class AssignmentType
{
    [Key]
    public Guid ID { get; set; }

    [Required]
    [StringLength(100)]
    public string Name { get; set; }        
}

Answer 1

试试这样的：

public IHttpActionResult Add(ReportingDTO data)
{            
    Reporting reporting = new Reporting { ID = Guid.NewGuid(), Date = DateTime.Now };                     

    foreach (Assignment assignment in data.Assignments)
    {
        if (assignment.ID != null)
        {
            _db.Assignments.Attach(assignment);
            _db.Entry(assignment).State = EntityState.Modified;
        }
        else 
        {
            assignment.ID = Guid.NewGuid();
        }

        if (assignment.AssignmentType != null)
        {
            assignment.AssignmentTypeID = assignment.AssignmentType.ID;
            assignment.AssignmentType = null;
        }
    }

    reporting.Assignment.AddRange(data.Assignments);

    foreach (Comment comment in data.Comments)
    {
        comment.ID = Guid.NewGuid();
        comment.AssignmentID = comment.Assignment.ID;
        comment.Assignment = null;
        comment.ReportingID = reporting.ID;
    }

    _db.Reporting.Add(reporting);
    _db.Comments.AddRange(data.Comments);
    _db.SaveChanges();

    return Ok(reporting);
}

您的所有Entry() 操作在您的上下文中都是无用的，除非您正在更新实体。在这种情况下，您必须在更新其状态之前Attach() 更新的实体。

另外，回答您最初的问题：EF 添加了一个新的AssignmentType，因为它丢失了现有的AssignmentType。每次请求命中时都会构造您的 Controller，因此每次也会构造您的 DbContext。当您创建新的 DbContext 时，它不知道以前的操作，例如返回 Assignment 或其他任何内容（如果您在查询期间指定 AsNoTracking() 也是这种情况）。换句话说，如果实体没有被 DbContext 跟踪，那么 EF 会尝试添加它。
为了防止这种情况，您可以（并且确实应该）使用外键而不是导航属性。或者你可以Attach()每个已知的实体（但如果你有外键就没用了）。对于多对多关系，您必须在添加/更新新实体之前查询数据库，否则，您将有达布隆。

或者，您也可以使用我所说的“链接表”。这只是一个介于两个主要课程之间的课程。 EF 已经为您的所有多对多关系执行此操作（检查您的数据库）。基本上，它是这样的：

public class A 
{
    public int Id {get; set;}
    public virtual List<AB> ABs { get; set; }
}

public class B
{
    public int Id {get; set;}
    public virtual List<AB> BAs {get; set; }
}

public class AB
{
    public int AId {get;set;}
    public virtual A {get; set;}

    public int BId {get;set;}
    public virtual B {get; set;}
}

通过这种方式，您可以使用_db.ABs.Add(new AB{AId = 1, BId = 2}) 添加一个新的 AB（或删除一个），而无需处理其余部分。