Accessing embedded documents in mongo-ruby-driver is as simple as using the dot notation:
result = @collection.find({'meta.authors': 'alex giamas'}).to_a
puts result
"_id"=>BSON::ObjectId('593c24443c8ca55b969c4c54'), "isbn"=>"201", "name"=>"Mastering MongoDB, 2nd Edition", "meta"=>{"authors"=>"alex giamas"}}
We need to enclose the key name in quotes ('') to access the embedded object, just as we need it for operations starting with $, such as '$set'.