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

What is this model? Is it a polymorphic relationship?

Rails should handle this...

class Foo < ActiveRecord::Base
  has_many :assignments, polymorphic: true
  
  scope :assigned, -> { assignments.any? }
end

...wait, you are looking for any that are assigned...let me retry that...

class Foo < ActiveRecord::Base
  has_many :assignments, polymorphic: true
  # this _should_ work
  scope :assigned, -> { joins(:assignments) }
  # but this is similar and includes a `where assignments.id is not null` with the join.
  scope :assigned, -> { joins(:assignments).merge(Assignments.where.not(id: nil)) }
end

Thanks for the feedback. Experimenting now... but it appears to be returning duplicate AssignableJob records.

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