create an array of arrays using mongoose

i have nodejs web app and my user structure has username, password, twitter, I want to add another thing which will be an array containing arrays. so like after adding three arrays in a user's array it will look like: [ {'a' :'b' , 'c':'d'} , {'a' :'b' , 'c':'d'}, {'a' :'b' , 'c':'d'}]