# -- Intervention Model --
class Intervention < ApplicationRecord

  belongs_to :father, class_name: "Intervention", foreign_key: "intervention_id", optional: true
  has_one :child, class_name: "Intervention", foreign_key: "intervention_id", dependent: :destroy

  scope :neither_father_nor_child, -> { where.missing(:father, :child) }

  def self.ransackable_scopes(auth_object = nil)
     %i[neither_father_nor_child]
  end
end

h = {:g=>{"0"=>{:neither_father_nor_child=>1}}}

Intervention.ransack(h) # => Ransack::Search<class: Intervention, base: Grouping <combinator: and>>
Intervention.ransack(h).result.to_sql # => SELECT \"interventions\".* FROM \"interventions\"

Intervention.ransack(neither_father_nor_child: 1) => # Ransack::Search<class: Intervention, scope: {"neither_father_nor_child"=>1}, base: Grouping <combinator: and>