A meteor is already talking to a mongodb. But you can use mongoose. You may have a 10 second delay with reactivity. Also, you will not be able to use it on the client.
Meteor already has methods for requesting / updating, etc. mongodb. But if you want, you can force the mongoose to:
Install the mongoose (npm install mongoose). And use it in your meteor code:
require = __meteor_bootstrap__.require; //to use npm require must be exposed. var mongoose = require('mongoose');
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