On 3 jul 2009, at 11:28, Jochem Berndsen wrote:
Chris Eidhof wrote:
I've something working that sort of does this. You define your
model in
the following way:
data User = User {name :: String, password :: String, age :: Int,
post
:: BelongsTo Post}
data Post = Post {title :: String, body :: String}
Then there's some boilerplate code (that ultimately will be
generated by
TH), and from that moment on you can do things like this:
test = do
conn <- connectSqlite3 "example.sqlite3"
runDB conn $ do
user <- fromJust <$> find typeUser 1
user' <- fillBelongsTo user relPost
return (post user')
By default, no relations will be fetched, but by doing the
fillBelongsTo
the user will be updated. I currently have support for new, update
and
find. All of this code is very alpha, and only works using HDBC and
Sqlite3, but still.
So in this example, both user and user' are of type User, but if I ask
for "post user", this is undefined?
I have done something similar as you, except that I filled the related
field with an unsafePerformIO fetching the related data from the
database.
No, it will never be undefined. The BelongsTo datatype is defined as
following:
data BelongsTo a = BTNotFetched | BTId Int | BTFetched (Int, a)
So either there is no information available (BTNotFetched), we know
the id (the foreign key, BTId) or we know the id and the value
(BTFetched). It is currently just a proof of concept, but for me, an
extend version of this will do. I think that almost every mapping from
Haskell datatypes to a RDBMS will be slightly awkward, this is my way
to find a balance ;)
-chris
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