这个问题是RavenDB: Why do I get null-values for fields in this multi-map/reduce index?的副产品，但我意识到，问题是另一个问题。
考虑我极其简化的域名，重写为电影租赁商店场景以进行抽象：
public class User
{
public string Id { get; set; }
}
public class Movie
{
public string Id { get; set; }
}
public class MovieRental
{
public string Id { get; set; }
public string MovieId { get; set; }
public string UserId { get; set; }
}
这是一本多对多的教科书。
我想要创建的索引是：
对于给定的用户，请给我一个数据库中每部电影的列表（暂时过滤/搜索）以及描述用户租借此电影的次数（或零）的整数。
基本上是这样的：
用户：
| Id |
|--------|
| John |
| Lizzie |
| Albert |
影：
| Id |
|--------------|
| Robocop |
| Notting Hill |
| Inception |
MovieRentals：
| Id | UserId | MovieId |
|-----------|--------|--------------|
| rental-00 | John | Robocop |
| rental-01 | John | Notting Hill |
| rental-02 | John | Notting Hill |
| rental-03 | Lizzie | Robocop |
| rental-04 | Lizzie | Robocop |
| rental-05 | Lizzie | Inception |
理想情况下，我想要一个查询索引，如下所示：
| UserId | MovieId | RentalCount |
|--------|--------------|-------------|
| John | Robocop | 1 |
| John | Notting Hill | 2 |
| John | Inception | 0 |
| Lizzie | Robocop | 2 |
| Lizzie | Notting Hill | 0 |
| Lizzie | Inception | 1 |
| Albert | Robocop | 0 |
| Albert | Notting Hill | 0 |
| Albert | Inception | 0 |
或以声明的方式：
但是，我无法找到一种方法来进行交叉连接。上面并将其保存在索引中。相反，我最初认为我在下面的操作中做到了正确，但它不允许我排序（参见测试失败）：
{＆＃34;不支持计算：x.UserRentalCounts.SingleOrDefault（rentalCount =＆gt;（rentalCount.UserId == value（UnitTestProject2.MovieRentalTests +＆lt;＆gt; c__DisplayClass0_0）.user_john.Id））。计数。您不能在RavenDB查询中使用计算（只允许使用简单的成员表达式）。＆＃34;}
我的问题基本上是：我怎么能 - 或者我可以 - 索引，我的要求是否满足？
以下是我提到的例子，它不符合我的要求，但这就是我现在所处的位置。它使用以下包（VS2015）：
packages.config
<?xml version="1.0" encoding="utf-8"?>
<packages>
<package id="Microsoft.Owin.Host.HttpListener" version="3.0.1" targetFramework="net461" />
<package id="NUnit" version="3.5.0" targetFramework="net461" />
<package id="RavenDB.Client" version="3.5.2" targetFramework="net461" />
<package id="RavenDB.Database" version="3.5.2" targetFramework="net461" />
<package id="RavenDB.Tests.Helpers" version="3.5.2" targetFramework="net461" />
</packages>
MovieRentalTests.cs
using System.Collections.Generic;
using System.Linq;
using NUnit.Framework;
using Raven.Client.Indexes;
using Raven.Client.Linq;
using Raven.Tests.Helpers;
namespace UnitTestProject2
{
[TestFixture]
public class MovieRentalTests : RavenTestBase
{
[Test]
public void DoSomeTests()
{
using (var server = GetNewServer())
using (var store = NewRemoteDocumentStore(ravenDbServer: server))
{
//Test-data
var user_john = new User { Id = "John" };
var user_lizzie = new User { Id = "Lizzie" };
var user_albert = new User { Id = "Albert" };
var movie_robocop = new Movie { Id = "Robocop" };
var movie_nottingHill = new Movie { Id = "Notting Hill" };
var movie_inception = new Movie { Id = "Inception" };
var rentals = new List<MovieRental>
{
new MovieRental {Id = "rental-00", UserId = user_john.Id, MovieId = movie_robocop.Id},
new MovieRental {Id = "rental-01", UserId = user_john.Id, MovieId = movie_nottingHill.Id},
new MovieRental {Id = "rental-02", UserId = user_john.Id, MovieId = movie_nottingHill.Id},
new MovieRental {Id = "rental-03", UserId = user_lizzie.Id, MovieId = movie_robocop.Id},
new MovieRental {Id = "rental-04", UserId = user_lizzie.Id, MovieId = movie_robocop.Id},
new MovieRental {Id = "rental-05", UserId = user_lizzie.Id, MovieId = movie_inception.Id}
};
//Init index
new Movies_WithRentalsByUsersCount().Execute(store);
//Insert test-data in db
using (var session = store.OpenSession())
{
session.Store(user_john);
session.Store(user_lizzie);
session.Store(user_albert);
session.Store(movie_robocop);
session.Store(movie_nottingHill);
session.Store(movie_inception);
foreach (var rental in rentals)
{
session.Store(rental);
}
session.SaveChanges();
WaitForAllRequestsToComplete(server);
WaitForIndexing(store);
}
//Test of correct rental-counts for users
using (var session = store.OpenSession())
{
var allMoviesWithRentalCounts =
session.Query<Movies_WithRentalsByUsersCount.ReducedResult, Movies_WithRentalsByUsersCount>()
.ToList();
var robocopWithRentalsCounts = allMoviesWithRentalCounts.Single(m => m.MovieId == movie_robocop.Id);
Assert.AreEqual(1, robocopWithRentalsCounts.UserRentalCounts.FirstOrDefault(x => x.UserId == user_john.Id)?.Count ?? 0);
Assert.AreEqual(2, robocopWithRentalsCounts.UserRentalCounts.FirstOrDefault(x => x.UserId == user_lizzie.Id)?.Count ?? 0);
Assert.AreEqual(0, robocopWithRentalsCounts.UserRentalCounts.FirstOrDefault(x => x.UserId == user_albert.Id)?.Count ?? 0);
var nottingHillWithRentalsCounts = allMoviesWithRentalCounts.Single(m => m.MovieId == movie_nottingHill.Id);
Assert.AreEqual(2, nottingHillWithRentalsCounts.UserRentalCounts.FirstOrDefault(x => x.UserId == user_john.Id)?.Count ?? 0);
Assert.AreEqual(0, nottingHillWithRentalsCounts.UserRentalCounts.FirstOrDefault(x => x.UserId == user_lizzie.Id)?.Count ?? 0);
Assert.AreEqual(0, nottingHillWithRentalsCounts.UserRentalCounts.FirstOrDefault(x => x.UserId == user_albert.Id)?.Count ?? 0);
}
// Test that you for a given user can sort the movies by view-count
using (var session = store.OpenSession())
{
var allMoviesWithRentalCounts =
session.Query<Movies_WithRentalsByUsersCount.ReducedResult, Movies_WithRentalsByUsersCount>()
.OrderByDescending(x => x.UserRentalCounts.SingleOrDefault(rentalCount => rentalCount.UserId == user_john.Id).Count)
.ToList();
Assert.AreEqual(movie_nottingHill.Id, allMoviesWithRentalCounts[0].MovieId);
Assert.AreEqual(movie_robocop.Id, allMoviesWithRentalCounts[1].MovieId);
Assert.AreEqual(movie_inception.Id, allMoviesWithRentalCounts[2].MovieId);
}
}
}
public class Movies_WithRentalsByUsersCount :
AbstractMultiMapIndexCreationTask<Movies_WithRentalsByUsersCount.ReducedResult>
{
public Movies_WithRentalsByUsersCount()
{
AddMap<MovieRental>(rentals =>
from r in rentals
select new ReducedResult
{
MovieId = r.MovieId,
UserRentalCounts = new[] { new UserRentalCount { UserId = r.UserId, Count = 1 } }
});
AddMap<Movie>(movies =>
from m in movies
select new ReducedResult
{
MovieId = m.Id,
UserRentalCounts = new[] { new UserRentalCount { UserId = null, Count = 0 } }
});
Reduce = results =>
from result in results
group result by result.MovieId
into g
select new
{
MovieId = g.Key,
UserRentalCounts = (
from userRentalCount in g.SelectMany(x => x.UserRentalCounts)
group userRentalCount by userRentalCount.UserId
into subGroup
select new UserRentalCount { UserId = subGroup.Key, Count = subGroup.Sum(b => b.Count) })
.ToArray()
};
}
public class ReducedResult
{
public string MovieId { get; set; }
public UserRentalCount[] UserRentalCounts { get; set; }
}
public class UserRentalCount
{
public string UserId { get; set; }
public int Count { get; set; }
}
}
public class User
{
public string Id { get; set; }
}
public class Movie
{
public string Id { get; set; }
}
public class MovieRental
{
public string Id { get; set; }
public string MovieId { get; set; }
public string UserId { get; set; }
}
}
}
答案 0 :(得分：1)
由于您的要求是针对给定用户＆＃34;，如果您真的只关注单个用户，则可以使用多地图索引执行此操作。使用Movies表本身生成基线零计数记录，然后在用户的实际MovieRentals记录中进行映射。
如果你真的需要所有与所有电影交叉的用户，我不相信有办法用RavenDB干净利落地完成这项工作，因为这会被视为reporting which is noted as one of the sour spots for RavenDB。
如果您真的想尝试使用RavenDB执行此操作，可以使用以下选项：
1）在DB中为每个用户和每部电影创建虚拟记录，并使用索引中的0记录。无论何时添加/更新/删除电影或用户，都应相应地更新虚拟记录。
2）根据请求自己在内存中生成零计数记录，并将该数据与RavenDB为非零计数返回的数据合并。查询所有用户，查询所有电影，创建基线零计数记录，然后对非零计数进行实际查询并在顶部进行分层。最后，应用分页/过滤/排序逻辑。
3）使用SQL复制包将用户，电影和MovieRental表复制到SQL并使用SQL进行此操作＆＃34;报告＆＃34;查询。