Difference between revisions of "CouchDB"

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= CouchDB =
 
 
[[Category:Packages]]
 
 
CouchDB Haskell package is the Haskell interface to the couchDB database software. I appreciate the efforts of Arjun Guha and Brendan Hickey to construct this interface. I think it is an improvement over the original and most convenient to use!
 
 
CouchDB is a document oriented datastorage system (with versions) which is geared towards replication. For more information read Anderson, Lehnardt and Slater's book "CouchDB - The definite guide" ([http://books.couchdb.org/relax/]).
 
 
Examples how to use the Haskell CouchDB interface are not easy to find on the web. I found only one [http://www.maztravel.com/haskell/mySqlToCouchDB.html], which is probably compiled with an slightly earlier version of CouchDB than the current 0.10.1.
 
 
I created this wiki page to make available the simple examples I coded to learn to use CouchDB and to report of some of the not-so-obvious traps beginners could fall into.
 
 
= Example: Store and retrieve notes =
 
 
The simple example I selected is the database backend of a "note" application (e.g. Tomboy or any other of the yellow paste-it notes look-alike). The first function is to store a note, as an example on how to store data in couch:
 
 
== Store note ==
 
 
Here the code for storing a note and retrieving it with the doc id:
 
<haskell>
 
{-# LANGUAGE DeriveDataTypeable
 
, ScopedTypeVariables #-}
 
 
module Notes1 where
 
 
import Database.CouchDB (getDoc, newDoc, runCouchDB', db, Rev(..), Doc)
 
import Data.Data (Data, Typeable)
 
 
import Text.JSON
 
import Text.JSON.Pretty (pp_value)
 
import Text.JSON.Pretty (render)
 
import Text.JSON.Generic (toJSON, fromJSON)
 
 
type Strings = [String] -- basic
 
 
data Note = Note {title, text :: String, tags :: Strings}
 
deriving (Eq, Ord, Show, Read , Typeable, Data) -- not yet necessary
 
 
------ copied from henry laxon
 
 
ppJSON = putStrLn . render . pp_value
 
 
justDoc :: (Data a) => Maybe (Doc, Rev, JSValue) -> a
 
justDoc (Just (d,r,x)) = stripResult (fromJSON x)
 
where stripResult (Ok z) = z
 
stripResult (Error s) = error $ "JSON error " ++ s
 
justDoc Nothing = error "No such Document"
 
 
--------------------------------
 
mynotes = db "firstnotes1"
 
 
n0 = Note "a59" "a1 text vv 45" ["tag1"]
 
 
n1 = Note "a56" "a1 text vv 45" ["tag1"]
 
n2 = Note "a56" "updated a1 text vv 45" ["tag1"]
 
 
n1j = toJSON n1 -- convNote2js n1
 
 
runNotes1 = do
 
(doc1, rev1) <- runCouchDB' $ newDoc mynotes n1j
 
putStrLn $ "stored note" ++ show doc1 ++ " revision " ++ show rev1
 
Just (_,_,jvalue) <- runCouchDB' $ getDoc mynotes doc1
 
ppJSON jvalue
 
 
jstuff <- runCouchDB' $ getDoc mynotes doc1
 
let d = justDoc jstuff :: Note
 
putStrLn $ "found " ++ show d
 
return ()
 
 
-- the output is:
 
--stored noteaa45700981408039346f9c8c73f8701f
 
-- revision 1-7fa1d1116e6ae0c1ee8d4ce89a701fdf
 
--{"_id": "aa45700981408039346f9c8c73f8701f",
 
-- "_rev": "1-7fa1d1116e6ae0c1ee8d4ce89a701fdf", "title": "a56",
 
-- "text": "a1 text vv 45", "tags": ["tag1"]}
 
--found Note {title = "a56", text = "a1 text vv 45", tags = ["tag1"]}
 
 
</haskell>
 
 
== Multiple functions to store and retrieve notes ==
 
Notes have a title, a text and a set of tags. We need functions to store a new note, to update the content of a note and to retrieve all notes with a given word in the title, in the text or as a tag; each of these functions return a list of pairs (doc_id, title) from which the correct one is selected and then retrieved with the doc_id.
 
 
I have decided that the notes have IDs produced by couchDB; this assures that changes, even changes in title, continue the same object with a new version.
 
 
<haskell>
 
 
{-# LANGUAGE DeriveDataTypeable
 
, ScopedTypeVariables #-}
 
 
module Notes2 (storeNewNote
 
, changeNoteContent
 
, updateNote
 
, deleteNote
 
, retrieveNote
 
, findNoteByTitle
 
, findNoteByTag
 
, findNoteByContent ) where
 
 
import Database.CouchDB
 
(rev, deleteDoc, CouchView(..), newView, Doc(..), isDocString,
 
queryView, getAndUpdateDoc, DB(..), getDoc, newDoc, db, doc,
 
newNamedDoc, runCouchDB', Rev(..))
 
-- , couchViewToJSON)
 
 
import Data.Data (Data, Typeable)
 
 
import Text.JSON
 
import Text.JSON.Pretty (pp_value)
 
import Text.JSON.Pretty (render)
 
import Database.CouchDB.JSON (jsonObject)
 
import Text.JSON.Generic (toJSON, fromJSON)
 
 
type Strings = [String] -- basic
 
 
-- from Henry Laxen's code:
 
 
type QueryViewResult = (Database.CouchDB.Doc,JSValue)
 
 
ppJSON = putStrLn . render . pp_value
 
 
justDoc :: (Data a) => Maybe (Database.CouchDB.Doc, Rev, JSValue) -> a
 
justDoc (Just (d,r,x)) = stripResult (fromJSON x)
 
where stripResult (Ok z) = z
 
stripResult (Error s) = error $ "JSON error " ++ s
 
justDoc Nothing = error "No such Document"
 
 
-----
 
docid_doc :: (Data a) => QueryViewResult -> (Doc, a)
 
docid_doc (d, x) = (d, stripResult . fromJSON $ x)
 
where stripResult (Ok z) = z
 
stripResult (Error s) = error $ "JSON error " ++ s
 
 
--------------------- some example data
 
 
mynotes = db mynotesString
 
mynotesString = "firstnotes1" -- the name of the couchdb
 
 
n0 = Note "a59" "a1 text vv 45" ["tag1", "tag2"]
 
 
n1 = Note "a56" "a1 text vv 45" ["tag1", "tag3"]
 
n2 = Note "a56" "updated a1 text vv 45" ["tag1", "tag2", "tag4"]
 
 
-------------------------------
 
 
data Note = Note {title, text :: String, tags :: Strings}
 
deriving (Eq, Ord, Show, Read , Typeable, Data) -- not yet necessary
 
 
storeNewNote :: DB -> Note -> IO ()
 
storeNewNote db note = do
 
let jnote = toJSON note -- the ID is set by couchdb
 
(doc, rev) <- runCouchDB' $ newDoc db jnote
 
putStrLn $ "stored note" ++ show doc ++ " " ++ show rev
 
return ()
 
 
 
changeNoteContent :: DB -> Doc -> Note -> IO ()
 
changeNoteContent db docid newnote = do
 
ret <- runCouchDB' $ getAndUpdateDoc db (docid) (updateNote newnote)
 
let rev = maybe (error "update did not succeed") id ret
 
putStrLn $ "changed " ++ show rev
 
return ()
 
 
updateNote :: Note -> JSValue -> IO JSValue
 
updateNote new old = return . const (toJSON new) $ old
 
 
deleteNote :: DB -> Doc -> Rev -> IO ()
 
deleteNote db id r = do
 
ret <- runCouchDB' $ deleteDoc db (id, r)
 
putStrLn $ "deleted " ++ show ret
 
return ()
 
 
retrieveNote :: DB -> Doc -> IO Note
 
retrieveNote db docid = do
 
ret <- runCouchDB' $ getDoc db docid -- :: Maybe (Doc, Rev, JSValue)
 
-- assumes the note title is the key
 
let n = justDoc ret :: Note
 
putStrLn $ "stored note" ++ show n
 
return n
 
 
findNoteByTitle :: DB -> String -> IO [(Doc, Note)]
 
findNoteByTitle db tit = do
 
putStrLn $ "search by title using view 'titles' " ++ show tit
 
ret :: [QueryViewResult] <- runCouchDB' $ do
 
queryView db (doc designdoc) (doc "bytitle") [("key", toJSON tit)]
 
putStrLn $ show ret
 
let ls = map docid_doc ret
 
putStrLn $ "result " ++ show ls
 
return ls
 
 
findNoteByTag :: DB -> String -> IO [(Doc, Note)]
 
findNoteByTag db ta = do
 
putStrLn $ "search by title using view 'titles' " ++ show ta
 
ret :: [QueryViewResult] <- runCouchDB' $ do
 
queryView db (doc designdoc) (doc "bytags") [("key", toJSON ta)]
 
putStrLn $ show ret
 
let ls = map docid_doc ret
 
putStrLn $ "result " ++ show ls
 
return ls
 
 
findNoteByContent :: DB -> String -> IO [(Doc, Note)]
 
findNoteByContent db word = do
 
putStrLn $ "search by title using view 'titles' " ++ show word
 
ret :: [QueryViewResult] <- runCouchDB' $ do
 
queryView db (doc designdoc) (doc "bywords") [("key", toJSON word)]
 
putStrLn $ show ret
 
let ls = map docid_doc ret
 
putStrLn $ "result " ++ show ls
 
return ls
 
 
--------------------------------------------------
 
 
titleView = "function(doc) { if (doc.title && doc.text && doc.tags) \
 
\ {emit(doc.title, doc._id); } }"
 
titleView2 = ViewMap "bytitle" titleView
 
 
 
tagView = "function(doc) { if (doc.title && doc.text && doc.tags) \
 
\ { var len=doc.tags.length; \
 
\ for(var i=0; i<len; i++) { \
 
\ var value = doc.tags[i]; \
 
\ emit(value, doc._id); } \
 
\ } \
 
\ }"
 
-- tried in temporary views of Futon
 
 
tagView2 = ViewMap "bytags" tagView
 
 
wordView = "function (doc) { if (doc.title && doc.text && doc.tags) \
 
\ { var words = doc.text.split (' '); \
 
\ var len=words.length; \
 
\ for(var i=0; i<len; i++) { \
 
\ var value = words[i]; \
 
\ emit(value, doc._id); } \
 
\ } \
 
\ }"
 
-- use ' for strings inside a haskell string
 
-- escapes are not removed when converting to JSON ?
 
 
wordView2 = ViewMap "bywords" wordView
 
 
--- setting views:
 
designdoc = "five" -- "six" -- change for each edit -- cannot update!
 
 
setViews dbstring = do
 
r <- runCouchDB' $ newView dbstring designdoc [titleView2, tagView2, wordView2]
 
-- inconsistency: here a string, not a DB type !
 
putStrLn $ "view stored"
 
return ()
 
----
 
 
run2 = do
 
storeNewNote mynotes n0
 
retrieveNote mynotes (doc "c4bf00e96e2446ce1508ba055e9b7ef6")
 
changeNoteContent mynotes (doc "c4bf00e96e2446ce1508ba055e9b7ef6") n2
 
retrieveNote mynotes (doc "c4bf00e96e2446ce1508ba055e9b7ef6")
 
return ()
 
 
--stored note68d5cfa3622c4586a7a1bfc695e72765 1-dee34e8ccb4ef3ceb9a7dcfb3d7cd20d
 
--stored noteNote {title = "a1234", text = "a1 text vv 45", tags = ["tag1"]}
 
--changed 2-53bfc49b385805020194a59b39fd5ffb
 
--stored noteNote {title = "a56", text = "updated a1 text vv 45", tags = ["tag1","tag2","tag4"]}
 
 
-- error when duplicate is not appropriate:
 
--stored note"\"*** Exception: src/Database/CouchDB/Unsafe.hs:80:10-63: Irrefutable pattern failed for pattern (Text.JSON.Types.JSObject errorObj)
 
 
run3 :: IO () = do
 
setViews mynotesString
 
return ()
 
 
run4 :: IO () = do
 
ls :: [(Doc, Note)] <- findNoteByTitle mynotes "a55"
 
putStrLn $ "found by title " ++ show ls
 
return ()
 
 
 
</haskell>
 
 
 
Notice that the syntax for retrieval has changed since the example code for "converting from MySQL to CouchDB" was written: no preceeding "/_design/" in the name of the view.
 
 
= A few suggestions for improvement in the interface
 
Most annoying is that design documents cannot be updated; a new set must be stored with a new name. A function updateView would be useful.
 
The type of newView is not consistent with the other calls - it requires the name of the couchDB as a String, not a DB type.
 
I used couchViewToJSON to test the views i wrote - it could be useful to export it.
 
 
 
= Coda =
 
I do not guarantee for the correctness of the code (of course). I hope it is useful. I invite others to contribute their examples or more complex codes so we can learn from each other.
 
 
I am currently working on using couchDB and interested to hear comments at frank at geoinfo dot tuwien dot ac dot at.
 

Revision as of 14:55, 6 February 2021