Esqueleto内部选择计数

Question

I have the following Entities :

Group
  name Text

GroupUser
  user UserId
  group GroupId

and I would like to do a query like this:

select g.* /* Everything from g */
     , count(gu.id) groupUsersCount 
  from Group g
  left outer join GroupUser gu on gu.groupId = g.id
group by g.id

Can it be done with Esqueleto ?

Answer 1

The esqueleto docs for groupBy contain good examples of how to use it.

Moreover, by reading through the Getting Started section, you'll see several example of queries including the equivalent of table.* :

do people <- select $
             from $ \person -> do
             return person

Putting the two together means something like this should work:

select $ from \(g `LeftOuterJoin` gh) -> do
  on (gu ^. GroupId ==. g ^. Id)
  groupBy (g ^. Id)
  return (g, countRows)