EF Attach时已存在的处理方式

如果我们在先前的步骤中读取过数据，如

var list = db.Model.ToList();

之后再，附加

var o = new Model { Id = 1 };

db.Model.Attach(o);

就会报，类似这样的错误

Attaching an entity of type 'efAutoDetach.Model' failed because another entity of the same type already has the same primary key value. This can happen when using the 'Attach' method or setting the state of an entity to 'Unchanged' or 'Modified' if any entities in the graph have conflicting key values. This may be because some entities are new and have not yet received database-generated key values. In this case use the 'Add' method or the 'Added' entity state to track the graph and then set the state of non-new entities to 'Unchanged' or 'Modified' as appropriate.

意思是说，在上下文中，已经有一个同样主键的东西了。

这样的错误通常在直接更新时遇到，不是先查出来再更新，而是直接new一个出来，附加上去更新

第一次查询后，本地的local中就已经有相应的实体了。如果我们把local中的实体detach了，那我们新new的对象就可以attch上去了。

Detach的扩展方法

public static void Detach<T>(this DbContext db, T obj) where T : class
{
    ObjectContext oc = ((IObjectContextAdapter)db).ObjectContext;
    oc.Detach(obj);
}

获取主键信息的扩展方法

public static IEnumerable<string> GetEntityKeys<T>(this DbContext db) where T : class
{
    ObjectContext oc = ((IObjectContextAdapter)db).ObjectContext;
    var keys = oc.CreateObjectSet<T>().EntitySet.ElementType.KeyProperties.Select(x => x.Name);
    return keys;
}

我们需要通过主键信息构建一个表达式树，用于从local中获取实体

private static Expression<Func<T, bool>> GetFindExp<T>(T obj, IEnumerable<string> keys) where T : class
{
    var p = Expression.Parameter(typeof(T), "x");

    var keyexps = keys.Select(x =>
    {
        var member = Expression.PropertyOrField(p, x);
        var objV = typeof(T).GetProperty(x).GetValue(obj);
        var eq = Expression.Equal(member, Expression.Constant(objV));
        return eq;
    }).ToList();

    if (keys.Count() == 1)
    {
        return Expression.Lambda<Func<T, bool>>(keyexps[0], new[] { p });
    }

    var and = Expression.AndAlso(keyexps[0], keyexps[1]);
    for (var i = 2; i < keyexps.Count; i++)
    {
        and = Expression.AndAlso(and, keyexps[i]);
    }
    return Expression.Lambda<Func<T, bool>>(and, new[] { p });
}

于是可以找到local中的

public static T FindLocal<T>(this DbContext db, T obj) where T : class
{
    var keys = db.GetEntityKeys<T>();
    var func = GetFindExp<T>(obj, keys).Compile();
    return db.Set<T>().Local.FirstOrDefault(func);
}

最后综合使用上面的几个方法

public static void DetachOther<T>(this DbContext db, T obj) where T : class
{
    var local = db.FindLocal(obj);
    if (local != null)
    {
        db.Detach(local);
    }
}

使用

var o = new Model { Id = 1 };
db.DetachOther(o);
db.Model.Attach(o);

在attach之前，先detachother

以上。

(科技责编：李霞飞 )