define_callback

# File activerecord/lib/active_record/associations/builder/collection_association.rb, line 30
    def self.define_callback(model, callback_name, name, options)
      full_callback_name = "#{callback_name}_for_#{name}"

      callback_values = Array(options[callback_name.to_sym])
      method_defined = model.respond_to?(full_callback_name)

      # If there are no callbacks, we must also check if a superclass had
      # previously defined this association
      return if callback_values.empty? && !method_defined

      unless method_defined
        model.class_attribute(full_callback_name, instance_accessor: false, instance_predicate: false)
      end

      callbacks = callback_values.map do |callback|
        case callback
        when Symbol
          ->(method, owner, record) { owner.send(callback, record) }
        when Proc
          ->(method, owner, record) { callback.call(owner, record) }
        else
          ->(method, owner, record) { callback.send(method, owner, record) }
        end
      end
      model.send "#{full_callback_name}=", callbacks
    end