c# - EF Core 添加相关实体(通用存储库)

Question

我有一个具有以下模型和一对多关系的 EF Core Db

public class AttributeType
{
    public int AttributeTypeId { get; set; }
    public string Name { get; set; }

    public List<Attribute> AttributeList { get; set; }
}

public class Attribute
{
    public int AttributeId { get; set; }
    public string Name { get; set; }

    public int AttributeTypeId { get; set; }
    public AttributeType AttributeType { get; set; }
}

如果我尝试以这种方式添加属性，它会起作用:

using (var db = new SampleDbContext())
            {
                var attributeType = db.AttributeTypes.FirstOrDefault();

                Attribute attribute = new Attribute
                {
                    Name = "bbb",
                    AttributeTypeId = attributeType.AttributeTypeId,
                    AttributeType = attributeType
                };
                var item = db.Attributes.Add(attribute);
                db.SaveChanges();
            }

(实际上，attributeType 将来自组合框，名称来自文本框，但现在让我们保持代码简单)

但是当我尝试使用通用方法时，如下面的代码不起作用

     AttributeType attributeType;

                using (var db = new SampleDbContext())
                {
                    attributeType = db.AttributeTypes.FirstOrDefault();
                }

                Attribute attribute = new Attribute
                {
                    Name = "eee",
                    AttributeType = attributeType,
                    AttributeTypeId = attributeType.AttributeTypeId
                };

                DbService.Insert(attribute);

 public static bool Insert<T>(T item) where T : class
        {
            try
            {
                using (var db = new SampleDbContext())
                {
                    db.Add(item);
                    db.SaveChanges();
                }
                return true;
            }
            catch (Exception exception)
            {
                Debug.WriteLine($"DbService Insert Exception: {exception}");
                return false;
            }

我得到以下异常

Exception thrown: 'Microsoft.EntityFrameworkCore.DbUpdateException' in Microsoft.EntityFrameworkCore.dll DbService Insert Exception: Microsoft.EntityFrameworkCore.DbUpdateException: An error occurred while updating the entries. See the inner exception for details. ---> Microsoft.Data.Sqlite.SqliteException: SQLite Error 19: 'UNIQUE constraint failed: AttributeTypes.AttributeTypeId'. at Microsoft.Data.Sqlite.Interop.MarshalEx.ThrowExceptionForRC(Int32 rc, Sqlite3Handle db) at Microsoft.Data.Sqlite.SqliteCommand.ExecuteReader(CommandBehavior behavior) at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.Execute(IRelationalConnection connection, String executeMethod, IReadOnlyDictionary2 parameterValues, Boolean closeConnection)
at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.ExecuteReader(IRelationalConnection connection, IReadOnlyDictionary2 parameterValues) at Microsoft.EntityFrameworkCore.Update.ReaderModificationCommandBatch.Execute(IRelationalConnection connection) --- End of inner exception stack trace --- at Microsoft.EntityFrameworkCore.Update.ReaderModificationCommandBatch.Execute(IRelationalConnection connection) at Microsoft.EntityFrameworkCore.Update.Internal.BatchExecutor.Execute(Tuple2 parameters)
at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChanges(IReadOnlyList1 entriesToSave) at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChanges(Boolean acceptAllChangesOnSuccess) at Microsoft.EntityFrameworkCore.DbContext.SaveChanges(Boolean acceptAllChangesOnSuccess) at UwpSamples.Services.DbService.Insert[T](T item)

但是如果我不传递 AttributeType 或者我将它设置为 null，就像下面的代码一样，它会起作用:

AttributeType attributeType;

        using (var db = new SampleDbContext())
        {
            attributeType = db.AttributeTypes.FirstOrDefault();
        }

        Attribute attribute = new Attribute
        {
            Name = "ddd",
            AttributeType = null,
            AttributeTypeId = attributeType.AttributeTypeId
        };

        DbService.Insert(attribute);

-

AttributeType attributeType;

        using (var db = new SampleDbContext())
        {
            attributeType = db.AttributeTypes.FirstOrDefault();
        }

        Attribute attribute = new Attribute
        {
            Name = "ddd",
            AttributeTypeId = attributeType.AttributeTypeId
        };

        DbService.Insert(attribute);

谢谢

Answer 1

您正在实例化两个独立的上下文。一个是您检索 AttributeType 的位置，存储库中执行插入的第二个上下文将遍历实体并认为 AttributeType 是新的并尝试插入它。我建议每个请求使用一个上下文(假设这是在网络请求之后)。有一些变通方法涉及将实体标记为未更改，但您每次都必须重复该变通方法，并且它对每个模型都非常具体。

通常，您要么使用容器或 IoC 框架为每个请求检索上下文，要么在存储库的构造函数中传递上下文。通过这种方式，您可以跨不同的存储库进行多次调用，并且更改跟踪将在调用之间进行。它还简化了在对不同存储库的调用之间打开事务范围的场景。