Clean result structures

Sometimes return types of query functions are not 100% compliant with business logic needs. To define result transformations, we often need fields for foreign keys. These fields are usually usesles in business logic code. Of course, we can make a trade-off, and keep result structures a little bit polluted with data access needs, but there is a cleaner way. Instead of defining foreign key directly:

type Comment = {
    id: int
    postId: int
    parentId: int
    content: string
    author: string
    createdAt: DateTime
    modifiedAt: DateTime option
    modifiedBy: string option
}

we define two structures:

type PostRelated<'t> = {
    postId: int
    related: 't
}

type Comment = {
    id: int
    parentId: int
    content: string
    author: string
    createdAt: DateTime
    modifiedAt: DateTime option
    modifiedBy: string option
}

and define transformations slightly different:

let buildCommentTree (cl: Comment PostRelated list) = 
    cl |> List.map (fun c -> c.related) |> buildTree

let getPostsWithComments: int -> DataContext -> Post list = 
    sql "select id, blogId, name, title, content, author, createdAt, modifiedAt, modifiedBy, status from post where blogId = @id;
     select c.id, c.postId, c.parentId, c.content, c.author, c.createdAt from comment c join post p on c.postId = p.id where p.blogId = @id"
>> join (fun p -> p.id) (fun c -> c.postId) (fun p cl -> { p with comments = buildCommentTree cl })
|> curry