Haskell Esqueleto - Maybe Keys don't bind on LeftOuterJoin

Question

I am with some trouble while making a join which uses a maybe foreign key .

My Models:

OrderItem
  productSnapshot ProductSnapshotId

ProductFlow
  productInstance ProductInstanceId
  orderItem OrderItemId Maybe

ProductSnapshot
  productInstance ProductInstanceId

ProductInstance
  code Text

And my joins:

type Entities = [(Entity OrderItem, Entity ProductSnapshot, Entity ProductInstance, Maybe (Entity ProductFlow))]

loadEntities :: OrderItemId -> Handler Entities
loadEntities oiId = runDB
  $ E.select
  $ E.from $ \(oi `E.InnerJoin` ps `E.InnerJoin` pi `E.LeftOuterJoin` pf) -> do
    E.on $ pf E.?. ProductFlowOrderItem E.==. E.just (oi ^. OrderItemId)
    E.on $ pi ^. ProductInstanceId E.==. ps ^. ProductSnapshotProductInstance
    E.on $ ps ^. ProductSnapshotId E.==. oi ^. OrderItemProductSnapshot
    E.where_ (oi ^. OrderItemId E.==. E.val oiId)
    return (oi, ps, pi, pf)

I tried the Esqueleto join showed above, but I got the following error:

Couldn't match type ‘Key OrderItem’ with ‘Maybe (Key OrderItem)’
    Expected type: EntityField OrderItem (Maybe (Key OrderItem))
      Actual type: EntityField OrderItem (Key OrderItem)
    In the second argument of ‘(^.)’, namely ‘OrderItemId’
    In the first argument of ‘E.just’, namely ‘(oi ^. OrderItemId)’

I think it is caused because I'm using a maybe foreign key. I tried other ways, and got strange type errors. I think I don't understand completely well how these joins work, so a brief explanation about E.^. , E.just and E.?. would improve my Esqueleto usage.

Answer 1

I managed to understand the problem and solve it myself, instead of

E.on $ pf E.?. ProductFlowOrderItem E.==. E.just (oi ^. OrderItemId)

use this

E.on $ pf E.?. ProductFlowOrderItem E.==. (E.just . E.just) (oi ^. OrderItemId)

It is necessary to make the types match, because we are using E.?. with an already Maybe value, so, we have to make the right side a Maybe (Maybe AType) , otherwise it would be wrong, and to make it I just used E.just . E.just E.just . E.just which will wrap two times.