Using MongoDB queries in Haskell

MongoDB is a nonrelational schemaless database. In this recipe, we will obtain all data from MongoDB into Haskell.

Getting ready

We need to install MongoDB on our local machine and have a database instance running in the background while we run the code in this recipe.

MongoDB installation instructions are located at http://www.mongodb.org. On Debian-based operating systems, we can use apt-get to install MongoDB, using the following command line:

$ sudo apt-get install mongodb

Run the database daemon by specifying the database file path as follows:

$ mkdir ~/db
$ mongod --dbpath ~/db

Fill up a "people" collection with dummy data as follows:

$ mongo
> db.people.insert( {first: "Joe", last: "Shmoe"} )

Install the MongoDB package from Cabal using the following command:

$ cabal install mongoDB

How to do it...

1. Use the OverloadedString and ExtendedDefaultRules language extensions to make the MongoDB library easier to use:

   {-# LANGUAGE OverloadedStrings, ExtendedDefaultRules #-}
   import Database.MongoDB

2. Define and implement main to set up a connection to the locally hosted database. Run MongoDB queries defined in the run function as follows:

   main :: IO ()
   main = do
       let db = "test"
       pipe <- runIOE $ connect (host "127.0.0.1")
       e <- access pipe master db run
       close pipe
       print e

3. In run, we can combine multiple operations. For this recipe, run will only perform one task, that is, gather data from the "people" collection:

   run = getData
   
   getData = rest =<< find (select [] "people") {sort=[]}

How it works...

A pipe is established by the driver between the running program and the database. This allows running MongoDB operations to bridge the program with the database. The find function takes a query, which we construct by evoking the select :: Selector -> Collection -> aQueryOrSelection function.

Other functions can be found in the documentation at http://hackage.haskell.org/package/mongoDB/docs/Database-MongoDB-Query.html.

See also

If the MongoDB database is on a remote server, refer to the Reading from a remote MongoDB server recipe to set up a connection with remote databases.