Yesod Esqueleto - How can I express selects with inner pagination?

Question

I am doing a paginated resource, which will require an inner select, which I've already designed in sql terms. It has the following structure:

select * 
  from (
    select w.*, d.distance
      from `Work` w
      inner join AdrDistance d on d.nhood2 = w.nhood
    where d.nhood1 = 1 -- this will be a variable
    order by d.distance
    limit 0, 10 -- this will be pagination
  ) w
  inner join WImage wi on wi.`work` = w.id

My entity definitions:

Work
  ...

WImage
  work WorkId
  url Text

AdrNhood
  city AdrCityId
  name Text maxlen=100
  lat Double
  lng Double

-- This is a view with a computed column I used for ordering
AdrDistance
  nhood1 AdrNhoodId
  nhood2 AdrNhoodId
  distance Distance -- type Distance = Int - in Meters

How can I define such a select in Esqueleto, which would resemble such structure (by doing one single query of course)?

Update

I tried to follow this path:

worksByNhood nId offset' limit' = 
  from $ \wi -> do
  (w, d) <- from $ \(w `InnerJoin` d) -> do
    on $ d ^. AdrDistanceNhood2 ==. w ^. WorkNhood
    where_ (d ^. AdrDistanceNhood1 ==. val nId)
    orderBy [asc (d ^. AdrDistanceDistance)]
    offset offset'
    limit limit'
    return (w, d)
  where_ (wi ^. WImageWork ==. w ^. WorkId)
  return (w, d ^. AdrDistanceDistance, wi)

But it didn't drive me to the correct solution. If someone can help me (even saying that I would be better doing several selects because what I am trying is not viable in Esqueleto), please, comment or answer my question.

I've updated the question, I did put wrong entity definitions earlier. — FtheBuilder, Apr 16 '16 at 15:04

score 0 · Answer 1 · answered Apr 18 '16 at 14:01

I have read this issue on github and I concluded Esqueleto wasn't designed to support selects in froms the way I was trying, so I did differently:

worksByNhood nId offset' limit' = do
  works <- select $ from $ \(w `InnerJoin` d) -> do
    on $ d ^. AdrDistanceNhood2 ==. w ^. WorkNhood
    where_ (d ^. AdrDistanceNhood1 ==. val nId)
    orderBy [asc (d ^. AdrDistanceDistance)]
    offset offset'
    limit limit'
    return (w, d ^. AdrDistanceDistance)
  works' <- forM works $ \(w@(Entity wId _), d) -> do
    images <- select $ from $ \wi -> do
      where_ (wi ^. WImageWork ==. val wId)
      return wi
    return (w, d, images);
  return works'

It is not exactly what I was looking for, but for now I will use it. If somebody have a better approach, please, tell me.

Yesod Esqueleto - How can I express selects with inner pagination?

1 Answers1