child.rb

class Child < ApplicationRecord
    has_one :outage, through: :relationship
    has_one :relationship
end

outage.rb

class Outage < ApplicationRecord
    has_many :outages, through: :relationships
end

relationship.rb

class Relationship < ApplicationRecord
  belongs_to :outage
  belongs_to :child
  validates_uniqueness_of :child_id, :scope => :outage_id
end

schema.rb

ActiveRecord::Schema.define(version: 20171113194934) do

  create_table "children", force: :cascade do |t|
    t.string "otn"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

  create_table "outages", force: :cascade do |t|
    t.string "htn"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

  create_table "relationships", force: :cascade do |t|
    t.integer "outage_id"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.integer "child_id"
    t.index ["child_id"], name: "index_relationships_on_child_id"
    t.index ["outage_id"], name: "index_relationships_on_outage_id"
  end

end