PostgreSQL where all in array

Question

What is the easiest and fastest way to achieve a clause where all elements in an array must be matched - not only one when using IN? After all it should behave

Answer 1

select id from conversations where not exists(
    select * from conversations_users cu 
    where cu.conversation_id=conversations.id 
    and cu.user_id not in(1,2,3)        
)

this can easily be made into a rails scope.