Question

我在RavenDB中存储了一组简单的对象：

public class Question
{
    public string Id { get; set; }
    public DateTime CreatedOn { get; set; }
    public ICollection<User> Supporters { get; set; }
    public ICollection<Answer> Answers { get; set; }
}

public class Answer 
{
    public string Id { get; set; }
    public bool IsOfficial { get; set; }
}

现在我想查询RavenDB给我一组问题，首先按照支持者的数量排序，然后按条件排序 - 如果问题有任何正式答案，并且在问题创建日期。所以我写了一个查询：

var questions = DocumentSession.Query<Question>().AsQueryable(); 
questions = questions
                .OrderByDescending(x => x.Supporters.Count)
                .ThenByDescending(x => x.Answers.Any(a => a.IsOfficial)) //EDIT: source of exception
                .ThenByDescending(x => x.CreatedOn)
                .Take(15);                
var result = questions.ToList();

引发异常：

System.InvalidCastException: Unable to cast object of type 'System.Linq.Expressions.MethodCallExpressionN' to type 'System.Linq.Expressions.MemberExpression'

查询在逻辑上是正确的，当我使用linq-to-objects时，只需将 .ToList（）添加到第一行：

var questions = DocumentSession.Query<Question>().Tolist().AsQueryable();
// next lines stay unchanged

由于性能问题，我不想这样做（此更改会强制所有问题在过滤前从数据库加载到内存中）。

如何在不影响性能的情况下使其正常运行？也许shell我定义了一个索引？它应该如何呢？

Answer 1

用于您的目的的自定义索引基本上将是您的类的重新创建，其中包含额外的字段（以及支持它的一些逻辑）。您似乎不想在当前类中添加更多字段，是否可以在项目中添加更多类？

以下是一个例子：

public class Question_WithAnyOfficial: AbstractIndexCreationTask<Question>
{
    public class Question_WithAnyOfficial()
    {
        Map = questions => from question in questions
                           // New Anonymous Type
                           select new
                           {
                                Id = question.Id,
                                CreatedOn = question.CreatedOn,
                                Supporters = question.Supporters,
                                Answers = question.Answers,
                                AnyOfficial = question.Answers.Where(a => a.IsOfficial).Any()
                           };
    }
}

然后你可以查询：

var questions = DocumentSession.Query<Question_WithAnyOfficial>()
                .OrderByDescending(x => x.Supporters.Count)
                .ThenByDescending(x => x.AnyOfficial)
                .ThenByDescending(x => x.CreatedOn)
                .Take(15)
                .ToList();

不要忘记在应用启动时您必须注册索引。

Answer 2

Raven不能支持LINQ查询中的计算，所以这应该有效（删除了问题子句）：

var questions = DocumentSession.Query<Question>()
                .OrderByDescending(x => x.Supporters.Count)
                //.ThenByDescending(x => x.Answers.Any(a => a.IsOfficial))
                .ThenByDescending(x => x.CreatedOn)
                .Take(15);                
var result = questions.ToList();

如果要包含该逻辑，则需要在类上使用名为AreAllAnswersOfficial的字段（或类似内容）。然后你可以把它放在子句中：

var questions = DocumentSession.Query<Question>()
                .OrderByDescending(x => x.Supporters.Count)
                .ThenByDescending(x => x.AreAllAnswersOfficial)
                .ThenByDescending(x => x.CreatedOn)
                .Take(15);                
var result = questions.ToList();

Answer 3

根据熊亚历山大的反应，我这样做了：

public class QuestionByAnyOfficial : AbstractIndexCreationTask<Question, QuestionByAnyOfficial.Result>
{
    public class Result
    {
        public string Id;
        public bool AnyOfficial;
        public int SupportersCount;
        public DateTime CreatedOn;
    }

    public QuestionByAnyOfficial()
    {
        Map = questions => from question in questions                               
                           select new
                              {
                                  Id = question.Id,                                              
                                  AnyOfficial = question.Answers.Any(a => a.IsOfficial),
                                  SupportersCount = question.Supporters.Count,
                                  CreatedOn = question.CreatedOn
                              };            
    }
}

var questionIds = DocumentSession.Query<QuestionByAnyOfficial.Result, QuestionByAnyOfficial>()
                       .OrderByDescending(x => x.SupportersCount)
                       .ThenByDescending(x => x.AnyOfficial)
                       .ThenByDescending(x => x.CreatedOn)
                       .Take(NumberOfQuestions)
                       .Select(x => x.Id);
var questions = DocumentSession.Load<Question>(questionIds);
var result = questions.ToList();

它有效，我相信它比我的原始版本更有效。如果它可以以更优雅的方式完成，我会感激任何想法。问候。

嵌套LINQ表达式的RavenDB解决方法

3 个答案: