C# 具有关系实体的LINQ连接查询

我使用的是ef core 2.1，我有以下简化实体，其中一个

帐户

映射到零个或多个

属性

对象：

public class Account
{
    public int Id { get; set; }
    public int LongId { get; set; }
    public List<Attribute> Attributes { get; set; }
}

public class Attribute
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Value { get; set; }
    public Account Account { get; set; }
}

我得到以下结果集：

id|objType
-----------
cejawq|ext
issokq|ext
cqlpjq|int
mbgzvi|int
wqwlff|ext
iedifh|null

我正在努力将其映射到高效的LINQ，以便生成的SQL远程生成结果集，并将数据发送回我可以投射到等效的匿名类型。我是否需要关心LINQ案例中的父对象？我在

属性.Value

列上没有索引

属性表包含以下数据：

Id|Name   |Value |AccountId
1 |uid    |cejawq|1
2 |objType|ext   |1
3 |uid    |issokq|2
4 |objType|ext   |2
5 |uid    |cqlpjq|3
6 |objType|int   |3
7 |uid    |mbgzvi|4
8 |objType|int   |4
9 |uid    |wqwlff|5
10|objType|ext   |5

由于EF Core不支持与内存序列的联接（但），因此您可以将查询分为两部分—一部分将数据服务器端（
[Attributes
到
[Attributes
联接）使用内存集合作为过滤器（SQL
通过LINQ
包含
方法），第二个将在内存中对db查询的结果执行左联接：

DbContext db = ...;
var uids = new [] { "cejawq", "issokq", "cqlpjq", "mbgzvi", "wqwlff", "iedifh" };

var dbQuery =
    from attr1 in db.Set<Attribute>()
    where attr1.Name == "uid" && uids.Contains(attr1.Value)
    join attr2 in db.Set<Attribute>()
    on new { AccountId = attr1.Account.Id, Name = "objType" }
    equals new { AccountId = attr2.Account.Id, attr2.Name }
    into attr2Group from attr2 in attr2Group.DefaultIfEmpty() // left outer join
    select new { uid = attr1.Value, objType = attr2.Value };

var query =
    from uid in uids
    join dbResult in dbQuery on uid equals dbResult.uid
    into dbResultGroup from dbResult in dbResultGroup.DefaultIfEmpty() // left outer join
    select new { uid, dbResult?.objType };

var result = query.ToList();

请为您的查询示例发布dbo.Attributes表。我添加了一些示例数据，说明了查询所利用的功能。我在回答中添加了模型和数据上下文类。我在属性类中添加了属性AccountId。我在尝试构建查询时注意到，不合并AccountId属性是一个错误这让我很难做到。我要到早上才能访问完整的数据集，但我计划尽快尝试一下。非常感谢您提供的详细帮助！太棒了，查询立即返回了生产数据的结果集。您帮了我很大的忙。
DbContext db = ...;
var uids = new [] { "cejawq", "issokq", "cqlpjq", "mbgzvi", "wqwlff", "iedifh" };

var dbQuery =
    from attr1 in db.Set<Attribute>()
    where attr1.Name == "uid" && uids.Contains(attr1.Value)
    join attr2 in db.Set<Attribute>()
    on new { AccountId = attr1.Account.Id, Name = "objType" }
    equals new { AccountId = attr2.Account.Id, attr2.Name }
    into attr2Group from attr2 in attr2Group.DefaultIfEmpty() // left outer join
    select new { uid = attr1.Value, objType = attr2.Value };

var query =
    from uid in uids
    join dbResult in dbQuery on uid equals dbResult.uid
    into dbResultGroup from dbResult in dbResultGroup.DefaultIfEmpty() // left outer join
    select new { uid, dbResult?.objType };

var result = query.ToList();