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

Based on @Alex Blakemore's answer, the equivalent Rails 4 scope on you Conversation class would be:

# Conversations exactly with users array
scope :by_users, -> (users) { 
                           self.by_any_of_users(users)
                             .group("conversations.id")
                             .having("COUNT(*) = ?", users.length) -
                           joins(:conversations_users)
                             .where("conversations_users.user_id NOT IN (?)", users)
}
# generates an IN clause
scope :by_any_of_users, -> (users) { joins(:conversations_users).where(conversations_users: { user_id: users }).distinct }

Note you can optimize it instead of doing a Rails - (minus) you could do a .where("NOT IN") but that would be really complex to read.