update_attributes & update do not call validations on parent association - ruby-on-rails

Question

So i have the following problem in my codebase.
In database there is data that doesn't satisfy the validations. (its not right I know, and will be fixed but the question remains why rails works like it does)
Calling save in create action will run validations on the parent object and wont save anything, however, calling the update or update_attributes in update action will not and the object will be saved with invalid parent.
Model:
class RussianDoll < ActiveRecord::Base
validates :name, :description, presence: true
has_and_belongs_to_many :parents,
class_name: 'RussianDoll',
join_table: :russian_dolls_russian_dolls,
foreign_key: :child_id,
association_foreign_key: :parent_id
has_and_belongs_to_many :children,
class_name: 'RussianDoll',
join_table: :russian_dolls_russian_dolls,
foreign_key: :parent_id,
association_foreign_key: :child_id
end
Controller:
class RussianDollsController < ApplicationController
def create
rd_params = russian_doll_params
rd_params[:parents] = RussianDoll.where(id: params[:parent_ids])
#russian_doll = RussianDoll.new(rd_params)
respond_to do |format|
if #russian_doll.save
format.html { render 'foo' }
format.xml { render 'bar' }
else
format.html { render action: 'new' }
format.xml { render xml: #russian_doll.errors }
end
end
end
def update
#russian_doll = RussianDoll.find(params[:id])
rd_params = russian_doll_params
rd_params[:parents] = RussianDoll.where(id: params[:parent_ids])
respond_to do |format|
if #russian_doll.update(rd_params)
format.html { render 'foo' }
format.xml { render 'bar' }
else
format.html { render action: 'edit' }
format.xml { render xml: #russian_doll.errors }
end
end
end
private
def russian_doll_params
params.require(:russian_doll).permit(:name, :description)
end
end
When you try to create a new RussianDoll (with name AND description) it wont save if the RussianDoll parent you are trying to associate it with doesnt have description set in database.
However if you create it without any parents, and then try to add a parent through the update action, then you will be able to set association to a RussianDoll entry that doesnt have description set (invalid entry).
As far as I know update should run all validations, same as the save action does but it doesn't seem to do that in this case.
Why do save validations work, but update ones don't?
Any help is appreciated!
[EDIT]
So I finally managed to find the root cause...
In create action we first create the #russian_doll with already set invalid parent in its association, so if we run .valid? on the object it will fail.
However in the update action, our #russian_doll is being first found in database, and before the .update(rd_params) object itself is not being changed, so running .valid? will return true, and update will manage to connect edited russian_doll with invalid parent.
What still bugs me, is that second time the update is called, #russian_doll will have invalid parent from a first update, but for some reason .valid? is still returning true, instead of failing.
It is like validations work for all nested entries only on object creation...