Concise Guide to Databases by Peter Lake & Paul Crowther

Author:Peter Lake & Paul Crowther
Language: eng
Format: epub, pdf
Publisher: Springer London, London

6.6 Using MongoDB’s db.collection.mapReduce() Method

As we saw in Chap. 5, MongoDB provides a number of built-in methods in the shell. We used the db.collection.find() method, for example, to search for records.

Another shell method provided is the db.collection.mapReduce() method which is a wrapper around the mapReduce command. We need to write a little bit of code to make this work for us in our environment—in effect creating our own extension to the Map-Reduce process.

Start by re-opening the MongoDB database. Assuming you inserted the Airport data in Chap. 5 you can jump straight to the Airports Database and we will be working with the AllAirports collection. If you did not do this you need to go to the end of the MongoDB tutorial in Chap. 5. The screen dump below reminds us how to do this before starting to build the functions required.

You would probably also need to remind yourself of what the data looks like (again, see Chap. 5). Our first Map Reduce exercise will count the number of Airports each country has.

Remembering that MapReduce is actually a two stage process we need to create first the map function and then the reduce function. In this case the AirportCount_map function adds the number 1 to the output against each instance of every Country Code. The reduce function will use the 1 for its count. The “emit” line is where we define what to output. The reduce function will then add up all the 1s for each Country Code. Reduce takes the output from Map and creates an Array. The word “this” is used to refer to “the collection currently being operated on”. When we call this function you will see that the collection is part of the call.

We have now created the functions (Fig. 6.4) required and the next step is to actually call the functions, using the mapReduce method that MongoDB provides. We have to tell the method which collection we are using, and this is what our map function will use as “this”.

Fig. 6.4Creating the MapReduce functions

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