Create a SelfJoin with has_many
Does the format provided in the Rails Guides not work for you?

# Model ------------------------------------------------------------------------
class Feedstock &lt; ApplicationRecord
  has_many :feedstocks,   foreign_key: "primary_feedstock_id"

  belongs_to :primary_feedstock, class_name: "Feedstock"
end

# Migration --------------------------------------------------------------------
class CreateFeedstocks &lt; ActiveRecord::Migration[5.0]
  def change
    create_table :feedstocks do |t|
      t.references :primary_feedstock, index: true
      t.timestamps
    end
  end
end


EDIT: So I'm still not clear on the difference between add_foreign_key and t.references. According to this comment on SO, t.references will not create a foreign key constraint, but that was written 5 years ago, and the part of the Guides that the commenter mentioned is no longer there. 

The API docs are abundantly clear about add_foreign_key creating a foreign key constraint at the database level; unfortunately, they're not so clear about t.references. 

In any case, you will have to decide for yourself how to design your application. I've heard some people say that depending on ActiveModel is ‘the Rails Way’, but I haven't seen that position stated anywhere in the official docs. Alternately, others will tell you that implementing these checks at the database level is more performant and reliable, and I can't argue with that. I think in the end, the choice comes down to how strictly you believe in Rails' convention-over-configuration philosophy, and whether you're willing to sacrifice a little abstraction for performance and reliability.

EDIT 2: It appears that add_foreign_key alone is not sufficient to create a table column. You'll need t.references either way, followed by either the foreign_key: true option or a separate add_foreign_key directive.