How does MongoDB index arrays?

When it comes to indexing arrays, MongoDB indexes each value of the array so you can query for individual items, such as “red.” For example:

> db.col1.save({'colors': ['red','blue']})
> db.col1.ensureIndex({'colors':1})

> db.col1.find({'colors': 'red'})
{ "_id" : ObjectId("4ccc78f97cf9bdc2a2e54ee9"), "colors" : [ "red", "blue" ] }
> db.col1.find({'colors': 'blue'})
{ "_id" : ObjectId("4ccc78f97cf9bdc2a2e54ee9"), "colors" : [ "red", "blue" ] }

For more information, check out MongoDB’s documentation on Multikeys: http://www.mongodb.org/display/DOCS/Multikeys