Difficult Rails ActiveModel Query - mysql

Question

I am building a Rails app which has a User model, joined to a Interests model through a has_many :through relationship using a UserInterests join model.
Given a set of interests, what I want to do is find all users who have selected at least one interest in that set, OR who have not selected any interests at all. I am thinking of something a bit like this:
users = users.joins(:user_interests).where(["COUNT(user_interests) = 0 OR user_interests.interest_id IN ?", interest_ids])
This raises a Mysql syntax error. Any ideas how to achieve this?
Many thanks

Answer 1

try this one,
users = User.joins(:user_interests).where("COUNT(user_interests) = 0 OR interest_id IN (?)", interest_ids)

Answer 2

class User < ActiveRecord::Base
scope :with_interests, ->(*interests) {
joins(:user_interests).where(user_interests: {interest_id: interests.flatten.compact.uniq})
}
scope :no_interests, -> {
where("not exists (select * from user_interests ui2 where ui2.user_id = users.id)")
}
end
interest = Interest.first
User.with_interests(interest) + User.no_interests

Answer 3

This worked for me - although it is not 100% ideal.
Add a user_interests_count field to users with a counter_cache
Search using a LEFT JOIN query:
users = User.joins("LEFT JOIN user_interests ON user_interests.user_id = users.id").where("(users.user_interests_count = ?) OR user_interests.interest_id IN (?)", 0, interest_ids)