.net – 我们如何在基于Dot Net的Azure移动服务中加载相关对象(Eager Loading)？

如果我有以下模型结构

public class QuestionItem: EntityData
{
    public string Content { get; set; }
    public bool IsAnswered { get; set; }
    public int NumberOfAnswers
    {
        //todo: make it computable
        get;
        set;
    }
    public UserItem By { get; set; }
    public string ById { get; set; }
    public string AtLocation { get; set; }
}

&安培;父母实体为

public class UserItem:EntityData
{

    public string UserName { get; set; }

    public string Gender { get; set; }

    public string BaseLocation { get; set; }

    public int Age { get; set; }

    public int Points { get; set; }

}

这确实生成了具有适当FK关系的DB表.但是在查询问题项列表时,QuestionItem.By属性(UserItem表的引用对象)为null.

通常这是通过在查询级别使用.Include方法来实现的,但我无法找到我应该在哪里做到这一点.

任何帮助表示赞赏.

Supreet

正如@JuneT所提到的,您需要从客户端发送$expand标头.原因是默认情况下,Entity Framework不会遍历对象图,因为这需要在数据库中进行连接,如果您不需要这样做,可能会对性能产生负面影响.

我在this blog post中提到的另一种替代方法是在服务器端使用过滤器为您添加该标头.例如,使用以下属性：

[AttributeUsage(AttributeTargets.Method, AllowMultiple = true)]
class ExpandPropertyAttribute : ActionFilterAttribute
{
    string propertyName;

    public ExpandPropertyAttribute(string propertyName)
    {
        this.propertyName = propertyName;
    }

    public override void OnActionExecuting(HttpActionContext actionContext)
    {
        base.OnActionExecuting(actionContext);
        var uriBuilder = new UriBuilder(actionContext.Request.RequestUri);
        var queryParams = uriBuilder.Query.TrimStart('?').Split(new[] { '&' }, StringSplitOptions.RemoveEmptyEntries).ToList();
        int expandIndex = -1;
        for (var i = 0; i < queryParams.Count; i++)
        {
            if (queryParams[i].StartsWith("$expand", StringComparison.Ordinal))
            {
                expandIndex = i;
                break;
            }
        }

        if (expandIndex < 0)
        {
            queryParams.Add("$expand=" + this.propertyName);
        }
        else
        {
            queryParams[expandIndex] = queryParams[expandIndex] + "," + propertyName;
        }

        uriBuilder.Query = string.Join("&", queryParams);
        actionContext.Request.RequestUri = uriBuilder.Uri;
    }
}

您可以装饰您的操作以强制扩展相关实体.

[ExpandProperty("By")]
public IQueryable<QuestionItem> GetAll() {
    return base.Query();
}