c# - EF 将多个表合并为一个 IQueryable

Question

我正在使用 OData(WebAPI 和 EF)查询数据库。现在我必须将三个表“合并”为一个结果。

我有 3 个表和一个界面，如下所示:

public class Authority : IAssociationEntity
{
    public string Name { get; set; }
    public int AuthorityId { get; set; }
}

public class Company : IAssociationEntity
{
    public string Name { get; set; }
    public int CompanyId { get; set; }
}

public class Organization : IAssociationEntity
{
    public string Name { get; set; }
    public int OrganizationId { get; set; }
}

public interface IAssociationEntity
{
    string Name { get; set; }
}

如您所见，这三个表之间有一些明显的相似之处，但出于某些原因，它们需要保留在单独的表中。我需要的是使用分页并按名称搜索所有三个，并将它们显示在同一个列表中以供用户使用。

我正在寻找在 SQL 中看起来像这样的东西

SELECT TOP 4 a.* FROM
(
    SELECT CompanyID, Name from Company WHERE Name = 'Bob'
    UNION
    SELECT OrganizationID, Name from Organization WHERE Name = 'Bob'
    UNION
    SELECT AuthorityID, Name from Authority WHERE Name = 'Bob'
) AS a

有没有办法把三张表合二为一IQueryable ？

我想将这三个表合并成一个 IQueryable<IAssociationEntity> .我确实需要使用接口(interface)(或可能是基类)并将结果作为 IQueryable对于我的 OData 实现。像这样的东西，但它不编译:

var query = db.Companies
    .Concat(db.Organizations)
    .Concat(db.Authorities);
IQueryable<IAssociationEntity> mergedTables = query.Cast<IAssociationEntity>();

// Here is an EXAMPLE usage.
// What I really need is to return the IQueryable<IAssociationEntity> for my OData.
var result = mergedTables.Where(x => x.Name == "Bob").OrderBy(x => x.Name).Skip(2).Take(10);

以及我对 odata Controller 的使用:

public class AssociationController : ODataController
{
    [EnableQuery]
    public override IQueryable<IAssociationEntity> Get(ODataQueryOptions<IAssociationEntity> q)
    {
        // return my IQueryable here...
    }
}

不用说，我不想在创建 IQueryable 时将整个表读入内存.实际上，我确实需要使用分页，因为这三个表中有几个表有数百万行。

最终解决方案如下:

  var query = db.Companies.Select(x => new AssociationEntity { Name = x.Name })
.Concat(db.Organizations.Select(x => new AssociationEntity { Name = x.Name }))
.Concat(db.Authorities.Select(x => new AssociationEntity { Name = x.Name }));
    return query;

并且在针对可查询对象执行时:

_query.Where(x => x.Name.Contains("M")).OrderBy(x => x.Name).Skip(10).Take(50).ToList();

生成的 SQL:

SELECT 
    [UnionAll2].[C1] AS [C1], 
    [UnionAll2].[Name] AS [C2]
    FROM  (SELECT 
        1 AS [C1], 
        [Extent1].[Name] AS [Name]
        FROM [dbo].[Company] AS [Extent1]
        WHERE [Extent1].[Name] LIKE N'%M%'
    UNION ALL
        SELECT 
        1 AS [C1], 
        [Extent2].[Name] AS [Name]
        FROM [dbo].[Organization] AS [Extent2]
        WHERE [Extent2].[Name] LIKE N'%M%'
    UNION ALL
        SELECT 
        1 AS [C1], 
        [Extent3].[Name] AS [Name]
        FROM [dbo].[Authority] AS [Extent3]
        WHERE [Extent3].[Name] LIKE N'%M%') AS [UnionAll2]
    ORDER BY [UnionAll2].[Name] ASC
    OFFSET 10 ROWS FETCH NEXT 50 ROWS ONLY 

Answer 1

您必须使用 Class 而不是 Interface IAssociationEntity。我将其命名为 AssociationEntity。

我已将您原来的TSQL 查询转换为:

SELECT TOP 4 a.* FROM
(
    SELECT CompanyID, Name from Company WHERE Name = 'Bob'
    UNION
    SELECT OrganizationID, Name from Organization WHERE Name = 'Bob'
    UNION
    SELECT AuthorityID, Name from Authority WHERE Name = 'Bob'
) AS a

Linq To Entity Query，如下所示。

var queryKey ="Bob";

var query = ((from c in db.Company  where (c.Name = queryKey) select new AssociationEntity { Name = c.Name }).Take(4))
.Concat((from o in db.Organization  where (o.Name = queryKey) select new AssociationEntity { Name = o.Name }).Take(4))
.Concat((from a in db.Authority  where (a.Name = queryKey) select new AssociationEntity { Name = a.Name }).Take(4));