"Could not deduce" Error When Sorting Many-To-One Collection Queries

[billksun]

I wanted to sort the results of a many-to-one relationship query. However, following the example for one-to-many relationships and modifying it for many-to-one, results in a "Could not deduce" error.

Here's an example code:

query @Comment |> fetch
  >>= pure . map (modify #postId (orderByAsc #createdAt))
  >>= collectionFetchRelated #postId

This results in this error:

Could not deduce (HasQueryBuilder Id' joinRegister0)
arising from a use of 'orderByAsc' from the context: (?context :: ControllerContext, ?modelContext :: ModelContext, ?theAction :: CommentsController)
bound by the type signature for ...

[mpscholten]

The code you've shared is basically saying "fetch all comments, with all their related posts ordered by created at". The problem is that each comment only belongs to a single post, so you cannot order the related posts by created at.

[billksun]

Ah, that makes much more sense than "Could not deduce...".

But how would I sort the results by a relation's field in a many-to-one relationship? As in I want to display a list of all comments along with their corresponding post title and post created timestamp, and sort the result by post created timestamp?

[mpscholten]

This query cannot be done with the IHP query builder directly. You could either write your own sql query using the sqlQuery function or reorder the results manually using a haskell sort function

[billksun]

Ah, thanks for confirming my suspicions. I think I will use a sortable JavaScript-based table instead.