I am trying to count the rows of an inner-join with Esqueleto (version 2.1.2.1). Unfortunately, my code doesn't compile and I don't understand why. I looked at the following examples of how to do this but couldn't figure out what I'm doing wrong: example1, example2.
My schema looks like this (simplified):
User
Game
state
Player
user UserId Maybe
game GameId
Users can register on the site to play games. You can also play without registering. Hence, there is a separate table Player. A game has a state. It can be Ongoing, or some form of game-over. I now want to count all ongoing games that a user is playing in.
The following SQL-query does that just fine (for a fixed userId of 1):
SELECT COUNT (*)
FROM Player INNER JOIN Game
ON Player.game = Game.id
WHERE Player.user = 1 AND game.state = "Ongoing"
However, the following Esqueleto query does not compile:
[count1] <- runDB $ E.select -- Line 25
$ E.from $ \(player `E.InnerJoin` game) -> do
E.on $ player^.PlayerGame E.==. game^.GameId
E.where_ $
player^.PlayerUser E.==. E.just (E.val userId) E.&&.
game^.GameState E.==. E.val MyGame.Ongoing
return (game, player)
return E.countRows -- Line 32
The error message reads like this:
Handler/ListUserGames.hs:25:23:
No instance for (E.SqlSelect (expr0 (E.Value a0)) r0)
arising from a use of ‘E.select’
The type variables ‘r0’, ‘expr0’, ‘a0’ are ambiguous
Note: there are several potential instances:
instance (E.SqlSelect a ra, E.SqlSelect b rb) =>
E.SqlSelect (a, b) (ra, rb)
-- Defined in ‘Database.Esqueleto.Internal.Sql’
instance (E.SqlSelect a ra, E.SqlSelect b rb, E.SqlSelect c rc) =>
E.SqlSelect (a, b, c) (ra, rb, rc)
-- Defined in ‘Database.Esqueleto.Internal.Sql’
instance (E.SqlSelect a ra, E.SqlSelect b rb, E.SqlSelect c rc,
E.SqlSelect d rd) =>
E.SqlSelect (a, b, c, d) (ra, rb, rc, rd)
-- Defined in ‘Database.Esqueleto.Internal.Sql’
...plus 13 others
In the expression: E.select
In the second argument of ‘($)’, namely
‘E.select
$ E.from
$ \ (player `E.InnerJoin` game)
-> do { E.on $ player ^. PlayerGame E.==. game ^. GameId;
E.where_
$ player ^. PlayerUser E.==. E.just (E.val userId)
E.&&. game ^. GameState E.==. E.val MyGame.Ongoing;
.... }’
In a stmt of a 'do' block:
[count1] <- runDB
$ E.select
$ E.from
$ \ (player `E.InnerJoin` game)
-> do { E.on $ player ^. PlayerGame E.==. game ^. GameId;
E.where_
$ player ^. PlayerUser E.==. E.just (E.val userId)
E.&&. game ^. GameState E.==. E.val MyGame.Ongoing;
.... }
Handler/ListUserGames.hs:32:32:
No instance for (E.Esqueleto query0 expr0 backend0)
arising from a use of ‘E.countRows’
The type variables ‘query0’, ‘expr0’, ‘backend0’ are ambiguous
Note: there is a potential instance available:
instance E.Esqueleto E.SqlQuery E.SqlExpr SqlBackend
-- Defined in ‘Database.Esqueleto.Internal.Sql’
In the first argument of ‘return’, namely ‘E.countRows’
In a stmt of a 'do' block: return E.countRows
In the expression:
do { E.on $ player ^. PlayerGame E.==. game ^. GameId;
E.where_
$ player ^. PlayerUser E.==. E.just (E.val userId)
E.&&. game ^. GameState E.==. E.val MyGame.Ongoing;
return (game, player);
return E.countRows }
However, the exact same query works if I remove the countRows. I.e. the following code compiles and does what I want it to do.
ongoing <- runDB $ E.select
$ E.from $ \(player `E.InnerJoin` game) -> do
E.on $ player^.PlayerGame E.==. game^.GameId
E.where_ $
player^.PlayerUser E.==. E.just (E.val userId) E.&&.
game^.GameState E.==. E.val MyGame.Ongoing
E.orderBy [E.desc $ game^.GameLastActionTime]
E.limit pagelen
E.offset $ max 0 $ (page1 - 1) * pagelen
return (game, player)
What am I doing wrong?
It turns out that the Esqueleto code above is correct. The error was in another part of the code where a lack of restrictions caused type-ambiguity.