Rails exclusion query with active record — Rails 3.1

Question

How would I use active record in this case:

I have two tables: Users, Admins

Admins can be users also and thus have a user_id in their table.

I'd like to Select all users that DO NOT have a user_id set in the admins table

NOTE: This is just an example case. If this was a real case then of course I wouldn't have two different tables for users and admins.

Edit: Again, I know the semantics of doing it like this is terrible db design -- but it's just an dummy case.

Answer 1

You can use :conditions to include a NOT IN :

User.find(:all, :conditions => ['user_id not in (?)', @admins.map( &:id )])

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OR you can code it into a User's scope ( aka named_scope prior to Rails 3.x ):

class User < ActiveRecord::Base
  scope :not_admin, lambda { |admins| { :conditions => ['user_id not in (?)', admins.select( &:id ).join( ',' )] }
end

And use it as:

User.not_admin( @admins )

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OR in order not to depend on @admins variable being passed you can use joins :

class User < ActiveRecord::Base
  scope :not_admin, joins( :admin ).where( :admins => { :user_id => nil } )
end