c# - 如何从 DBEntityEntry 中获取外键值？

Question

我正在尝试使用 Entity Framework 在保存时创建任何更改的审核日志。到目前为止，它运行良好，使用以下代码存储对每个字段所做的所有更改:

foreach (string propertyName in dbEntry.OriginalValues.PropertyNames)
{
    // For updates, we only want to capture the columns that actually changed
    if (!object.Equals(dbEntry.OriginalValues.GetValue<object>(propertyName), dbEntry.CurrentValues.GetValue<object>(propertyName)))
    {
        result.Add(new AuditLog()
        {
            UserID = UserId,
            EventDateUTC = changeTime,
            EventType = "M",    // Modified
            TableName = tableName,
            RecordID = primaryKey.ToString(),
            ColumnName = propertyName,
            OriginalValue = dbEntry.OriginalValues.GetValue<object>(propertyName) == null ? null : dbEntry.OriginalValues.GetValue<object>(propertyName).ToString(),
            NewValue = dbEntry.CurrentValues.GetValue<object>(propertyName) == null ? null : dbEntry.CurrentValues.GetValue<object>(propertyName).ToString()
        });
    }
}

我面临的问题是如何获取属于该对象的任何外键的值。例如:我有一个与一系列查找表有关系的车辆对象，例如变速箱、模型等。如果这些值发生变化，审计表将存储更改后的 ID，但我想存储实际值。

在这种情况下有没有办法获取外键值？

Answer 1

好吧...这是一个老问题，但我花了最后一个时间来解决这个问题，因为我有完全相同的要求。也许有更简单的方法，但这是我使用的代码:

您的原始代码，为我的目的稍作修改(RecordID 始终是一个 int)，并调用新方法来计算新值

foreach (string propertyName in dbEntry.OriginalValues.PropertyNames)
            {
                // For updates, we only want to capture the columns that actually changed
                if (!Equals(dbEntry.OriginalValues.GetValue<object>(propertyName), dbEntry.CurrentValues.GetValue<object>(propertyName)))
                {
                    var newVal = getNewValueAsString(dbEntry, tableName, propertyName);

                    result.Add(new AuditLog
                                {
                                    UserID = currentUser.ID,
                                    Timestamp = changeTime,
                                    EventType = EventType.Modified,
                                    TableName = tableName,
                                    RecordID = dbEntry.OriginalValues.GetValue<int>(keyName),
                                    ColumnName = propertyName,
                                    OriginalValue = dbEntry.OriginalValues.GetValue<object>(propertyName) == null ? null : dbEntry.OriginalValues.GetValue<object>(propertyName).ToString(),
                                    NewValue = newVal
                                }
                        );
                }
            }

名为“IsName”的新属性

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
public class IsNameAttribute : Attribute
{

}

使用 IsName 属性标记外键模型的“名称”属性(请注意，如果找不到，代码将默认为名为“名称”的属性)

[Required]
[IsName]
public string Name { get; set; }

还有繁重的代码

 private string getNewValueAsString(DbEntityEntry dbEntry, string tableName, string propertyName)
    {
        var fkVal = getForeignKeyValue(tableName, propertyName, dbEntry.CurrentValues.GetValue<object>(propertyName));
        return fkVal != null ? fkVal.ToString()
                            : (dbEntry.CurrentValues.GetValue<object>(propertyName) == null ? null
                                : dbEntry.CurrentValues.GetValue<object>(propertyName).ToString());
    }

    private object getForeignKeyValue(string tableName, string propertyName, object foreignKeyID)
    {
        // if this property is part of a foreign key, we need to instead look that up and store the value of the
        // foreign key

        // first get all the foreign keys in the system
        var workspace = ((IObjectContextAdapter)this).ObjectContext.MetadataWorkspace;
        var items = workspace.GetItems<AssociationType>(DataSpace.CSpace);
        if (items == null) return null;
        var fk = items.Where(a => a.IsForeignKey).ToList();
        // now we look into the FK attributes and find that the "To Role" is out current table, and the
        // "To Property" is out current property. The underscore is a bit of an assumption that the foreign
        // key name built by EF will be ENTITY_BLAH_BLAH
        var thisFk = fk.Where(x => x.ReferentialConstraints[0].ToRole.Name.StartsWith(tableName + "_"))
            .FirstOrDefault(x => x.ReferentialConstraints[0].ToProperties[0].Name == propertyName);
        // if fkname has no results, this is not a foreign key and we are done
        if (thisFk == null) return null;

        // Now that we know the foriegn key, we need to lookup the Name value in the other table

        // find the assembly
        var assembly = Assembly.GetCallingAssembly();
        // build the type for the foreign key entity
        // e.g. if the current entity is Task, and the property is StatusID, we are 
        // getting the "TaskStatus" type with reflection
        // "User" class is an object in the Models namespace - you could just hardcode the string if you want
        var foreignKeyType = assembly.GetType(typeof(User).Namespace + "." +
                  thisFk.ReferentialConstraints[0].FromRole.GetEntityType().Name);

        // get the DbSet, same as: "(new DBContext()).EntityName"
        var fkSet = Set(foreignKeyType);
        // and find the row in that table
        var fkItem = fkSet.Find(foreignKeyID);

        // find the first column marked with the "IsName" attribute, otherwise default to "Name"
        var nameColProperty = foreignKeyType.GetProperties()
            .FirstOrDefault(p => p.GetCustomAttributes(typeof(IsNameAttribute), false).Any());
        string nameCol = "Name";
        if (nameColProperty != null) nameCol = nameColProperty.Name;
        var nameColProperty2 = fkItem.GetType().GetProperty(nameCol);
        if (nameColProperty2 == null) return null;

        // get the value
        var fkValue = nameColProperty2.GetValue(fkItem, null);

        // and now, my brain hurts
        return fkValue;
    }