Arel subquery condition

assignable_job.rb

class AssignableJob < ApplicationRecord
  has_many :assignments, as: :record, dependent: :destroy

  scope :assigned, -> do
    subquery = Assignment.where(record_type: name).where(Assignment.arel_table[:record_id].eq(arel_table[:id])).select(Assignment.arel_table[Arel.star].count)
    where "(#{subquery.to_sql}) > 0" # TODO: move to Arel & remove string interpolation
  end
end

assignment.rb

class Assignment < ApplicationRecord
  belongs_to :record, polymorphic: true
end

Adding distinct produces what I'm after, but I worry about performance as the table grows.

class AssignableJob < ApplicationRecord
  scope :assigned, -> { joins :assignments }
end

AssignableJob.assigned # returns duplicate records
AssignableJob.assigned.distinct # works