CouchDB

CouchDB

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" ([1]).

Examples how to use the Haskell CouchDB interface are not easy to find on the web. I found only one [2], 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:

 
{-# 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"]}

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.

{-# 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 ()

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.