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

What about something like this -

class ModelA < ActiveRecord::Base
  has_many model_B
end

class ModelB < ActiveRecord::Base
  belongs_to model_A

  before_save :set_attribute

  def set_attribute
    self.value = '123'
  end
end

hmmm... I updated gist.

I would like to have an attribute in model_b to be set by a value that is only in model_a.

So why not -

class ModelA < ActiveRecord::Base
  has_many model_B
end

class ModelB < ActiveRecord::Base
  belongs_to model_A

  before_save :set_attribute

  def set_attribute
    self.value = a.some_value
  end
end

OIC. have model_b get its value from model_a... I have this working.

I was wondering why I can just do all this setting right from model_a (these are tightly linked models)

You certainly can do the setting from model_a. Both of the patterns suggested will work.

I prefer the latter because I think it's a more "proper" place for the responsibility of setting that value to lie. But that's more of a "code design" conversation than just a question of what will work.

I agree with you, I would rather have one place to set these value instead.

ok, i might have something else going on in the code preventing what I want to do.

i will spin up a fresh rails project and see if i still have the same problem.

thanks for your help!

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.