db.cities.remove({});  // clears the collection

db.cities.insert( {name: "Mumbai", continent: "Asia", population: 20400000} );
db.cities.insert( {name: "New York", continent: "North America", population: 21295000} );
db.cities.insert( {name: "London", continent: "Europe", population: 8580000} );
db.cities.insert( {name: "Rome", continent: "Europe", population: 2715000} );
db.cities.insert( {name: "Melbourne", continent: "Australia", population: 3900000} );
db.cities.insert( {name: "San Francisco", continent: "North America", population: 5780000} );
db.cities.insert( {name: "San Jose", continent: "North America", population: 7354555} );
db.cities.insert( {name: "Tokyo", continent: "Asia", population: 13185502} );
db.cities.insert( {name: "Hong Kong", continent: "Asia", population: 7234800} );

db.cities.find();

or

db.cities.find({});

db.cities.find({},{name: 1});
db.cities.find({},{_id: 0});
db.cities.find({},{name: 1, _id: 0});

db.cities.find({continent: "Asia"},{_id: 0});
db.cities.find({population: 7354555},{_id: 0});
db.cities.find({population: {$lt: 7354555}},{_id: 0});

db.cities.find({continent: {$in: ["Australia", "Asia"]}},{_id: 0});

db.cities.find({$or: [{continent: "Australia"},
                              {population: {$gt: 20000000}}]},{_id: 0});

db.cities.update({name: "San Francisco"},{$set: {university: "Stanford"}});
db.cities.find({name: "San Francisco"},{_id: 0});
db.cities.find({university: {$exists: true}},{_id: 0});

var c = db.cities.find();

while(c.hasNext()) {
        var curr = c.next();
        print(tojson(curr));
}

original_id = ObjectId();

db.universities.insert({
      _id: original_id,
      name: "Harvard"});

db.cities.insert( {name: "Boston",
                   continent: "North America", 
                   population: 636479,
                   university_id: original_id} );

var theCity = db.cities.find({name: "Boston"})[0];
db.universities.find({_id: theCity.university_id});

db.comments.drop();