Today I Learned Notes to self about software development

    Rails: Association Callbacks

    #rails
    31 Mar 2026

    TIL has_many associations have life cycle events you can define callbacks on.

    Available callbacks:

    • before_add
    • after_add
    • before_remove
    • after_remove

    They are added on the association (like with dependent: :destroy or counter_cache: true).

    class Author < ApplicationRecord
  has_many :books, before_add: :check_limit

  private
    def check_limit(_book)
      if books.count >= 5
        errors.add(:base, "Cannot add more than 5 books for this author")
        throw(:abort)
      end
    end
end

    If a before_add callback throws :abort, the object does not get added to the collection.

    If a before_remove callback throws :abort, the object does not get removed from the collection.

    You can make the value an Array to run multiple:

    class Author < ApplicationRecord
  has_many :books, before_add: [:check_limit, :calculate_shipping_charges]
  # ...
end

    Rails passes the object being added or removed to the callback for you to use.

    class Author < ApplicationRecord
  has_many :books, before_add: [:check_limit, :calculate_shipping_charges]

  # book will be the new book being created
  def calculate_shipping_charges(book)
    weight_in_pounds = book.weight_in_pounds || 1
    shipping_charges = weight_in_pounds * 2

    shipping_charges
  end
end

    These callbacks are called only when the associated objects are added or removed through the association collection.

    # Triggers `before_add` callback
author.books << book
author.books = [book, book2]

# Does not trigger the `before_add` callback
book.update(author_id: 1)

    This makes it more niche than regular model callbacks but it’s still pretty neat.

    Rails guide

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