blogs_controller.rb

class BlogsController < ApplicationController
  before_filter :setup
  
  def replace
    # For some reason we need only mark for destructions old article with name: "My First Article"
    blog.articles.each(&:mark_for_destruction)
  
    # params[:articles_attributes] => { name: "My First Article" }
    blog.articles.build(params[:articles_attributes])
    blog.save! # => raise exception!
  end
  
  private

  def blog
    @blog ||= Blog.find(params[:id]
  end
  
  def setup
    blog.articles.create! name: 'My First Article'
  end
end

create_blog_and_articles.rb

class CreateBlogsAndArticles < ActiveRecord::Migration
  def change
    create_table :blogs do |t|
    end

    create_table :articles do |t|
      t.string :name, null: false
      t.references :blog
    end
    
    add_index :articles, [:name, :blog_id], unique: true
  end
end

models.rb

class Article < ActiveRecord::Base
  #validates :name, uniqueness: true
end

class Blog < ActiveRecord::Base
  has_many :articles, autosave: true
end