Rails query tips

Exist

滿酷的用法之前沒想過

```ruby
# Post
scope :famous, ->{ where("view_count > ?", 1_000) }
# User
scope :without_famous_post, ->{
  where(_not_exists(Post.where("posts.user_id = users.id").famous))
}
def self._not_exists(scope)
  "NOT #{_exists(scope)}"
end
def self._exists(scope)
  "EXISTS(#{scope.to_sql})"
end
```

Subqueries

原來 rails 會自己 construct 這類的 sql

https://medium.com/@apneadiving/active-records-queries-tricks-2546181a98dd