How do I do a "contains" query with f# query expressions? - f#

Question

How do I do a query expression similar to a SQL IN-query?
I'm trying to do something along these lines:
let customerNumbers = set ["12345"; "23456"; "3456"]
let customerQuery = query {
for c in dataContext.Customers do
where(customerNumbers.Contains(c.CustomerNumber))
select c
}
But I'm getting an error:
System.NotSupportedException: Method 'Boolean Contains(System.String)' has no supported translation to SQL.
Looking at the documentation for query expressions at http://msdn.microsoft.com/en-us/library/hh225374.aspx I should use another query for the contains part but this code doesn't work, the example is broken:
// 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 does in fact not contain any "Contains" method.
I have also tried:
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
}
But this gives this error message:
System.NotSupportedException: Local sequence cannot be used in LINQ to SQL implementations of query operators except the Contains operator
I noticed after some more testing that the following also works fine in addition to Gene's suggestion:
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

The problem I believe comes from the way F# Set implements method Contains. It belongs to ICollection interface and this fact somehow upsets LINQ-to-SQL query builder.
If you explicitly force your Contains into the extension method of IEnumerable territory everything gets OK:
let customerNumbers = set ["12345"; "23456"; "3456"]
let customerQuery = query {
for c in dataContext.Customers do
where((customerNumbers :> IEnumerable<string>).Contains(c.CustomerNumber))
select c
}
Or, equivalently, you can add non-LINQ-to-SQL query
let idQuery = query { for id in customerNumbers do select id }
which has no problems with enumerating set yielding seq<string> and then use it for Contains as
....
where (idQuery.Contains(c.CustomerNumber))
....
Or, to begin with, you may keep your customerNumbers as seq:
let customerNumbers = set ["12345"; "23456"; "3456"] |> Set.toSeq
and use it as intuition prompts:
....
where(customerNumbers.Contains(c.CustomerNumber))
....