如何使用f#查询表达式执行"包含"查询？

Question

如何执行类似于SQL IN查询的查询表达式？

我试图沿着这些方向做点什么:

let customerNumbers = set ["12345"; "23456"; "3456"]
let customerQuery = query {
    for c in dataContext.Customers do
    where(customerNumbers.Contains(c.CustomerNumber))
    select c
}

但我收到一个错误:

System.NotSupportedException: Method 'Boolean Contains(System.String)' has no supported translation to SQL.

在http://msdn.microsoft.com/en-us/library/hh225374.aspx上查看查询表达式的文档我应该对contains部分使用另一个查询,但是这段代码不起作用,示例被破坏了:

// Select students where studentID is one of a given list.
let idQuery = query { for id in [1; 2; 5; 10] do select id }
query { 
    for student in db.Student do
    where (idQuery.Contains(student.StudentID))
    select student
}

idQuery确实不包含任何"Contains"方法.

我也尝试过:

let customerNumbers = set ["12345"; "23456"; "3456"]
let customerQuery = query { 
    for c in dataContext.Customers do
    where (query { for x in customerNumbers do exists (c.CustomerNumber=x)})
    select r
}

但是这会给出以下错误消息:

System.NotSupportedException: Local sequence cannot be used in LINQ to SQL implementations of query operators except the Contains operator

经过一些测试,我注意到除了Gene的建议之外,以下内容也可以正常工作:

let customerNumbers = set ["12345"; "23456"; "3456"]
query {
    for customer in dataContext.Customer do
    where (query { for x in customerNumbers do contains customer.CustomerNumber})
    select customer
}

Answer 1

我认为问题来自F#Set实现方法的方式Contains.它属于ICollection接口,这个事实以某种方式扰乱了LINQ-to-SQL查询构建器.

如果你明确强制你Contains进入IEnumerable领土的扩展方法,一切都会好起来的:

let customerNumbers = set ["12345"; "23456"; "3456"]
let customerQuery = query {
    for c in dataContext.Customers do
    where((customerNumbers :> IEnumerable<string>).Contains(c.CustomerNumber))
    select c
}

或者,等效地,您可以添加非LINQ-to-SQL查询

let idQuery = query { for id in customerNumbers do select id }

枚举set屈服没有问题,seq<string>然后用它Contains作为

....
where (idQuery.Contains(c.CustomerNumber))
....

或者,首先,你可以保持你customerNumbers的seq:

let customerNumbers = set ["12345"; "23456"; "3456"] |> Set.toSeq

并将其用作直觉提示:

....
where(customerNumbers.Contains(c.CustomerNumber))
....