`exists?` does a predicate-style select for this. If we end up iterating over `member?` somewhere, we can add the `include`

```ruby
2.0.0p247 :005 > u.memberships.where(organization: o).exists?
  Membership Exists (4.6ms)  SELECT 1 AS one FROM "memberships" WHERE "memberships"."user_id" = $1 AND "memberships"."organization_id" = 1 LIMIT 1  [["user_id", 1]]
```