haskell - Representing sum types in sql with Persistent, and Esqueleto joins -

i have been trying figure out sensible way represent haskell sum types in sql backend using persistent.

my target haskell data type along lines of

data widget = foowidget int | barwidget t.text  data helement = helement     { name   :: t.text    , widget :: widget    } 

i modeling these using following persistent data types:

element   name    t.text  foo   elementid elementid   size      int  bar   elementid elementid   colour    t.text 

there ever either foo or bar each element, never both.

i want use left outer join select elements , corresponding foo or bar. esqueleto expression is:

select $  $ \(elem `leftouterjoin` foo `leftouterjoin` bar) -> on (just (elem ^. elementid) ==. foo ?. fooelementid) on (just (elem ^. elementid) ==. bar ?. barelementid) return (elem, foo, bar) 

however, when execute code, error:

user error (postgresql.withstmt': bad result status fatalerror (("pgres_fatal_error","error:  missing from-clause entry table 

if remove second join, giving:

select $  $ \(elem `leftouterjoin` foo) -> on (just (elem ^. elementid) ==. foo ?. fooelementid) return (elem, foo) 

the code runs without error. i'm sure it's obvious can't see doing wrong.

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edit: found problem was; docs:

note order of on clauses reversed! you're required write ons in reverse order because helps composability (see documentation of on more details).

the following code works (the order of on expressions reversed):

select $ $ \(elem `leftouterjoin` foo `leftouterjoin` bar) -> on (just (elem ^. elementid) ==. bar ?. barelementid) on (just (elem ^. elementid) ==. foo ?. fooelementid) return (elem,foo,bar) 

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thanks,

michael

adding answer suggested waldheinz

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i found problem was; docs:

note order of on clauses reversed! you're required write ons in reverse order because helps composability (see documentation of on more details).

the following code works (the order of on expressions reversed):

select $ $ \(elem `leftouterjoin` foo `leftouterjoin` bar) -> on (just (elem ^. elementid) ==. bar ?. barelementid) on (just (elem ^. elementid) ==. foo ?. fooelementid) return (elem,foo,bar) 

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regards,

michael