How to recover a table from a database context via reflection in SqlProvider?

Question

I'm trying to have a powerful client-side querying API, where the client can specify which table to run the query in and the conditions to the query. Of course that's no replacement to full-blown LINQ or SQL, it's just so the JS client can make complex queries on tables, without joins.

Now, most of it is done, as below. The client sends me a JSON-serialized Predicate (which I deserialize with JSON.NET), and I can easily compose it into a query.

module Query =
    open System.Linq
    open FSharp.Data.Sql.Common
    open FSharpComposableQuery

    type Predicate =
    | All
    | Greater of string * System.IComparable
    | GreaterEq of string * System.IComparable
    | Lesser of string * System.IComparable
    | LesserEq of string * System.IComparable
    | Equal of string * obj
    | Diff of string * obj
    | And of Predicate * Predicate
    | Or of Predicate * Predicate
    | Not of Predicate

    let satisfies (table : IQueryable<SqlEntity>) =
        <@ fun p -> query {
           for c in table do
           if p c
           then yield c
          } @>

    let rec eval t =
        match t with
            | All                   -> <@ fun _ -> true @>
            | Greater (column, n)   -> <@ fun (c : SqlEntity) -> c.GetColumn column > n @>
            | GreaterEq (column, n) -> <@ fun c -> c.GetColumn column >= n @>
            | Lesser (column, n)    -> <@ fun c -> c.GetColumn column < n @>
            | LesserEq (column, n)  -> <@ fun c -> c.GetColumn column <= n @>
            | Equal (column, n)     -> <@ fun c -> c.GetColumn column = n @>
            | Diff (column, n)      -> <@ fun c -> c.GetColumn column <> n @>
            | And (p1, p2)          -> <@ fun c -> (%eval p1) c && (%eval p2) c @>
            | Or (p1, p2)           -> <@ fun c -> (%eval p1) c || (%eval p2) c @>
            | Not p                 -> <@ fun c -> not((%eval p) c) @>

    let predicate = Or (Equal ("myColumn", "myValue"), Equal ("myColumn", "myOtherValue"))
    let t = query {
        for c in <someDataProvidedTable> do
        select (c :> SqlEntity)
    }
    let result = query { yield! (%satisfies t) (%eval predicate) } |> Seq.toArray

module DataLayer =
    open FSharp.Data.Sql
    open FSharp.Data.Sql.Common
    let [<Literal>] ConnectionString = "Data Source=" + __SOURCE_DIRECTORY__ + @"/db.sqlite3;Version=3"
    type Sql = SqlDataProvider<
                ConnectionString = ConnectionString,
                DatabaseVendor = Common.DatabaseProviderTypes.SQLITE,
                IndividualsAmount = 1000,
                UseOptionTypes = true >
    let ctx = Sql.GetDataContext()
    let db = ctx.Maint

All that is left to parameterize is... the table. DataLayer.db has table objects equivalent to my tables, and my functions accept these table objects. But I want the client to be able to send me the table as a string, and I inspect DataLayer.db to retrieve a table from it.

Now, I know I could solve this by having a dictionary from string to table object. But that would make the whole construct less usable (we're generating the schema and the client from an external DSL). Of course, I have tried DataLayer.db.GetType().InvokeMethod, but I get the following exception no matter what the parameters are.

System.MissingMethodException: Method 'FSharp.Data.Sql.Runtime.SqlDataContext.FsmIssue' not found.
  at System.RuntimeType.InvokeMember (System.String name, System.Reflection.BindingFlags bindingFlags, System.Reflection.Binder binder, System.Object target, System.Object[] providedArgs, System.Reflection.ParameterModifier[] modifiers, System.Globalization.CultureInfo culture, System.String[] namedParams) [0x008a0] in <dca3b561b8ad4f9fb10141d81b39ff45>:0 
  at System.Type.InvokeMember (System.String name, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object target, System.Object[] args) [0x00000] in <dca3b561b8ad4f9fb10141d81b39ff45>:0 
  at <StartupCode$FSI_0062>.$FSI_0062.main@ () [0x0001a] in <906a7cff96cb466bbad0babb35abf8de>:0 
  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
  at System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00038] in <dca3b561b8ad4f9fb10141d81b39ff45>:0 
Stopped due to error

So, is anyone able to help me?

Answer 1

You may use define function like getTable below, which will return IQueryable for given context and table fullname:

open FSharp.Data.Sql
open System.Reflection
open FSharp.Data.Sql.Common
open System.Linq

type DB = SqlDataProvider< 
              ConnectionString = "Data Source=/Path/to/db",
              DatabaseVendor = Common.DatabaseProviderTypes.SQLITE,
              IndividualsAmount = 1000,
              UseOptionTypes = true>

let getTable ctx (tableFullName: string) =
  let m = 
    ctx.GetType().GetMethod("FSharp-Data-Sql-Common-ISqlDataContext-CreateEntities", BindingFlags.NonPublic ||| BindingFlags.Public ||| BindingFlags.Instance)

  m.Invoke(ctx,[| tableFullName |]) :?> IQueryable<SqlEntity> 

[<EntryPoint>]
let main _ = 
  let ctx = DB.GetDataContext()

  getTable ctx "Main.Test" |> Seq.iter ( fun x-> printfn "%A" x.ColumnValues)

  0 

note that tableFullName is DB specific, if you look at: FSharp.Data.Sql.Schema.Table source code you notice it's based on Schema and TableName where record members are set in specific providers:

• MS Access
• MS SQL
• MySQL
• ODBC
• Oracle
• PostgeSQL
• SQLite

EDIT: simpler way to toubleshot/find actual full table name

In case of doubts/problems with table full name you can rely on DB.dataContext which utilizes full name for variety of database objects.

UPDATE: question in comments regarding accessing Create

similarly to accessing all records, create can be accessed via reflection as:

let create ctx (tableFullName: string) (data: (string * obj) seq) =
  let m = 
    ctx.GetType().GetMethod("FSharp-Data-Sql-Common-ISqlDataContext-CreateEntity", BindingFlags.NonPublic ||| BindingFlags.Public ||| BindingFlags.Instance)

  let e = m.Invoke(ctx,[| tableFullName |]) :?> SqlEntity
  e._State <- Created
  e.SetData data
  e.DataContext.SubmitChangedEntity e
  e


[<EntryPoint>]
let main _ = 
  let ctx = DB2.GetDataContext()

  seq { yield "test",232L:>obj } |> create ctx "Main.Test" |> ignore

  ctx.SubmitUpdates()
  0 

this particular example utilizes (string * obj) seq as parameter, as seen in FSharp.Data.Sql.SqlDesignTime in create3. It can be adjusted to manner used in create1 or create2, or any other required.