Typesafe dynamic filtering/ordering/paging

この issue ではリポジトリのファイルやテストが指定されていません。まず、ここで説明されている SQL... 生成型のパスを見つけ、出力列がどのように公開されるかを追跡してください。対象となる単一 SELECT クエリが、型安全な By 値、並べ替え、ページング、および適用された句を保持するコマンドをサポートすれば完了です。

Right now there is no way to dynamically order queries.

This makes the library unsuitable for applications that let the user sort a paged table of records.
That's a lot of applications!

This should be solvable without giving up type safety. Here's the design I have in mind:

Any command that consists of a single SELECT statement which is not known to be a single-row SELECT should be dynamically orderable.

e.g.

type MyQuery = SQL<"select Column1, 1+1 as Column2 from SomeTable">

This means that it gets the following extra stuff in its generated type:

• A nested type MyQuery.By with a private constructor and an overrided ToString().
• Static getters MyQuery.By.Column1 and MyQuery.By.Column2, which return instances of MyQuery.By
• A static method MyQuery.By.OfString("Column1") which checks that the passed string is one of the statically known output column names. MyQuery.By.OfString(MyQuery.By.Column2.ToString()) should work.
• A static OrderBy method taking a MyQuery.By and a DU Ascending|Descending and returning a MyQuery.OrderedQuery
• MyQuery.OrderedQuery has an instance method ThenBy taking another by+direction, Page taking limit+offset
• MyQuery.OrderedQuery has an instance Command method which is like the regular MyQuery.Command(param1 = ...) method, but includes the orderings applied so far

Hypothetical usage:

type MyQuery = SQL<"select Column1, 1+1 as Column2 from SomeTable where Name like @name">

let example (conn : ConnectionContext) =
    MyQuery
        .OrderBy(MyQuery.By.Column1, Ascending)
        .ThenBy(MyQuery.By.Column2, Descending)
        .Page(25, 50)
        .Command(name = "%a%")
        .Execute(conn)

I think it would be OK to limit to one ThenBy clause. Orderings more complicated than that don't make much sense to generate dynamically.

Thoughts?