model_A.rb

has_many model_B

before_save set_model_b_attributes

def set_model_b_attributes
  self.bs.each do |b|
    b.value = self.value
  end
end

model_B.rb

belongs_to model_A, inverse_of: model_B

class ModelA < ActiveRecord::Base
  has_many model_B
  before_save set_model_b_attributes

  def set_model_b_attributes
    self.bs.each do |b|
      b.update_value(self.value)
    end
  end
end

class ModelB < ActiveRecord::Base
  belongs_to model_A

  def update_value(new_val)
    self.value = new_val
    self.save!
  end
end

I will try that. The app has a lot more going on so I will try a few things in a fresh project instead first.

@brycesenz

Got this working! (model_a setting model_b.attribute)

I think I was trying to write to model_b too late in the whole save process (or after model_b validated and basically frozen for changes).

Thanks for working through this with me. It definitely helped to know that this was not 'impossible' to do.