检查轨道中是否存在记录

Question

I have a people table used to track user online activities. I'd like to check if any user from a group is online or not with following query:

Person.where(:person_id => candidates, :online => true).present?

notes: candidates present the group of people; online attribute will be true if user is online

I realize such query is not efficient enough because it returns all the qualified users then check presence. Is there any better way to do the same thing more efficiently?

Thank

Answer 1

Using a COUNT query would be more efficient:

Person.where(:person_id => candidates, :online => true).count

You can then just compare with 0 :

Person.where(:person_id => candidates, :online => true).count > 0

Answer 2

you could do something like:

people = Person.select(:id, :online).where(id: candidate_ids)

then unless people.blank? loop through people and check whether they are online and do whatever you need to do with the people who are online. Using select doesn't load the entire object, only the things you need. Then when you say:

person.online

It's not a query, you're just getting it from an array. As a bonus, if you only have an individual user, .select also works with .find.