Rails how to find records where the has_many relation exists?

Question

In the below association, I want to collect all Users who don't have any projects->

class User <  ActiveRecord::Base    
  has_many :projects, :foreign_key => :user_id  
end

class Projects < ActiveRecord::Base 
  belongs_to :user, :foreign_key => "user_id"
end

From User model, how can I get all the users that do not have any projects? I tried using includes and join but didn't get the expected result

Answer 1

You could try:

User.where.not(id: Project.pluck(:user_id).uniq)

Breaking it down:

Project.pluck(:user_id).uniq

will give you an array of user_ids from your projects. Essentially, users with projects.

Then:

User.where.not(id: Project.pluck(:user_id).uniq)

returns users who have an id that is not in the array of users with projects.