MutiKeys

Introduction

Mongo provides some functionality that is useful for text search and tagging.

Multikeys (Indexing Values in an Array)

The Mongo multikey feature can automatically index arrays of values. Tagging is a good example of where this feature is useful. Suppose you have an article object/document which is tagged with some category names:

obj = {
  name: "Apollo",
  text: "Some text about Apollo moon landings",
  tags: [ "moon", "apollo", "spaceflight" ]
}

and that this object is stored in db.articles. The command

db.articles.ensureIndex( { tags: 1 } );

will index all the tags on the document, and create index entries for "moon", "apollo" and "spaceflight" for that document.

You may then query on these items in the usual way:

> print(db.articles.findOne( { tags: "apollo" } ).name);
Apollo

The database creates an index entry for each item in the array. Note an array with many elements (hundreds or thousands) can make inserts very expensive. (Although for the example above, alternate implementations are equally expensive.)

Text Search

It is fairly easy to implement basic full text search using multikeys. What we recommend is having a field that has all of the keywords in it, something like:

{ title : "this is fun" ,
  _keywords : [ "this" , "is" , "fun" ]
}

Your code must split the title above into the keywords before saving.  Note that this code (which is not part of Mongo DB) could do stemming, etc. too. (Perhaps someone in the community would like to write a standard module that does this...)