Rails ActiveRecord Scope that is the "opposite" of another scope, or is users that "lack" a property

Question

I have a model for user.rb, in which I define a scope for admins, which is users that have the role of admin through a permissions table.

has_many :permissions
has_many :roles, :through => :permissions

The scope works like this:

scope :admins, joins(:permissions).merge(Permission.admin_permissions)

I'd also like to make a scope called non-admins or something like that, which is all users that do NOT have the admin role.

What's the easiest way to do this?

Answer 1

An easy way, which would not be performant for a lot of users:

admin_user_ids  = User.admins.map(&:id)
non_admin_users = User.all.reject { |u| admin_user_ids.include?(u.id) }

Answer 2

If you want to have an inverted SQL query, you will have to do it yourself manually. There is no built-in ActiveRecord solution.

scope :admins, joins(:permissions).merge(Permission.where("permissions.admin = true"))
scope :non_admins, joins(:permissions).merge(Permission.where("permissions.admin = false"))

If there are a lot of scopes or they are complex, consider excluding them by id:

User.where("id not in (?)", User.admins.pluck(:id))

# or if you are already using admin records
admins = User.admins
User.where("id not in (?)", admins.map(&:id))

Depending on number of rows and complexity of the original query, this could be slower or faster than the previous way.