Parametric LINQ query

Question

This is another take on accessing dynamic objects in F# There I'm using let y = x.Where(fun x -> x.City ="London").Select("new(City,Zip)") to parametrize the query and extract the necessary items. These would correspond to columns in an SQL query, and be represented by a property of the datacontext. This is the part that I would like to pass in as a parameter.

type Northwind = ODataService<"http://services.odata.org/Northwind/Northwind.svc">
let db = Northwind.GetDataContext()
let query2 = query { for customer in db.Customers do
                     select customer}  |> Seq.toArray
let qryfun (x:Northwind.ServiceTypes.Customer) =
    query { for x in query2 do
            select (x.City,x.CompanyName,x.Country)}

Basically I would like to pass in not only x but also x.*. As I'm accessing one database that is fixed, I can factor out x. However I now have 40 small functions extracting the different columns. Is it possible to factor it out to one function and pass the property as an argument? So sometimes I extractx.City but other times x.Country. I have tried using quotations however cannot splice it properly and maybe that is not the right approach.

I think you can just return tuple from the query like select (customer, customer.City) and your qryfun will accept tuple as a parameter Since all variety of customer properties dont have static type, you either use tuples for intermediate data flow between functions/transformations or define your own record type and map it in your query — paulik
– paulik, Commented Apr 11, 2016 at 15:31
@paulik thx. I'm not sure that is doable. E.g. let qryfun (x, x.column) = query { for x in query2 do select (x.column)} where x is the DB table (Northwind.ServiceTypes.Customer) and x.column is a Column (or field) in the Customer table, it could be x.City, x.Country, etc.... then I would call qryfun (Northwind.ServiceTypes.Customer, Northwind.ServiceTypes.Customer.City) The 2nd argument is tricky because Northwind.ServiceTypes.Customer.City belongs to each record not to the table. If you take a look at the referenced question, I bult it from a string ("new(City, Country)") — s952163
– s952163, Commented Apr 11, 2016 at 23:21

kvb · Accepted Answer · 2016-04-11 21:44:46Z

7

Regarding quotation splicing, this works for me:

open System.Linq

type record = { x:int; y:string }

let mkQuery q =
    query {
        for x in [{x=1;y="test"}].AsQueryable() do
        select ((%q) x)
    }

mkQuery <@ fun r -> r.x, r.y @>
|> Seq.iter (printfn "%A")

answered Apr 11, 2016 at 21:44

kvb

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