IHP - How to use fetchRelated with fetchOneOrNothing?

Question

This is my schema:

Posts:
id
roomsId


Rooms:
id

RoomEvents:
id
roomId
userId
created_at

The query I'm writing is:

action MyAction { .. } = do
.
.
roomEvent <- query @RoomEvent
  |> filterWhere (#roomId, roomId)
  |> orderBy #createdAt
  |> fetchOneOrNothing
  >>= fetchRelated #userId

But this is throwing following error:

Web/Controller/Posts.hs:150:21: error:
    • Could not deduce (FromRow fetchModel0)
        arising from a use of ‘fetchRelated’
      from the context: (?context::ControllerContext,
                         ?modelContext::ModelContext, ?theAction::PostsController)
        bound by the type signature for:
                   action :: (?context::ControllerContext,
                              ?modelContext::ModelContext, ?theAction::PostsController) =>
                             PostsController -> IO ()
        at Web/Controller/Posts.hs:57:5-10
      The type variable ‘fetchModel0’ is ambiguous
The type variable ‘fetchModel0’ is ambiguous
      These potential instances exist:
        instance Database.PostgreSQL.Simple.FromField.FromField a =>
                 FromRow (Only a)
          -- Defined in ‘Database.PostgreSQL.Simple.FromRow’
        instance FromRow Activity
          -- Defined at build/Generated/Types.hs:412:10
        instance FromRow ActivityPostFile
          -- Defined at build/Generated/Types.hs:802:10
        ...plus 64 others
        ...plus one instance involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    • In the second argument of ‘(>>=)’, namely ‘fetchRelated #userId’

Does this mean that instances generated from other tables is interfering with this query? I've tried using fetchRelatedOrNothing and maybeFetchRelatedOrNothing too.

UPDATE: I was putting incorrect type in my View which led the type system to infer an incorrect type. Changing the type in View, immediately fixed the issue.

score 1 · Accepted Answer · answered Nov 19 '21 at 06:25

Using maybeFetchRelatedOrNothing should work here:

roomEvent <- query @RoomEvent
  |> filterWhere (#roomId, roomId)
  |> orderBy #createdAt
  |> fetchOneOrNothing
  >>= maybeFetchRelatedOrNothing #userId

What was the error when you tried this?

Also the |> orderBy #createdAt requires a created_at column in the room_events table, but it's not listed as part of your schema. Maybe this could also affect the issue?

IHP - How to use fetchRelated with fetchOneOrNothing?

1 Answers1