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 often 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
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
content: string
author: string
createdAt: DateTime
modifiedAt: DateTime option
modifiedBy: string option
}
and define transformations slightly different:
let getPostsWithComments: int -> Post list AsyncDb =
sql "select id, blogId, name, title, content, author,
createdAt, modifiedAt, modifiedBy, status
from post
where blogId = @id;
select c.id, c.postId, c.content, c.author, c.createdAt
from comment c join post p on c.postId = p.id
where p.blogId = @id"
>> AsyncDb.map (join (fun p -> p.id)
(fun c -> c.postId)
(fun p cl -> { p with comments = cl |> Seq.map (fun c -> c.related) }))
This approach can be used with convention-based transformations too, but you have to mark a wrapper type (i.e. PostRelated<’t>) with the interface IChildObject<'t>:
type PostRelated<'t> = {
postId: int
related: 't
}
interface IChildObject<'t> with
member this.Child = this.related
and use it when defining transformation:
let getPostsWithComments: int -> Post list AsyncDb =
sql "select id, blogId, name, title, content, author,
createdAt, modifiedAt, modifiedBy, status
from post
where blogId = @id;
select c.id, c.postId, c.content, c.author, c.createdAt
from comment c join post p on c.postId = p.id
where p.blogId = @id"
>> AsyncDb.map (join<_, PostRelated<Comment>>)