accepts_nested_attributes_for has_many and new association with ids.

gistfile1.txt

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', github: 'rails/rails'
    gem 'arel', github: 'rails/arel'
    gem 'sqlite3'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

require 'active_record'
require 'minitest/autorun'
require 'logger'

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts do |t|
  end

  create_table :comments do |t|
    t.integer :post_id
  end
end

class Post < ActiveRecord::Base
  has_many :comments
  accepts_nested_attributes_for :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

class BugTest < Minitest::Test
  def test_association_stuff
    comment = Comment.create!
    post = Post.new(comments_attributes:{"0"=>{"id"=>comment.id}})
    post.save!
    assert_equal 1, post.comments.count
    assert_equal post.id, Comment.first.post.id
    assert_equal comment.id,post.comments.first.id
    assert_equal 1, Comment.count
  end
end

it seems a similar issue was raised on http://stackoverflow.com/questions/18835422/rails-4-nested-attributes-couldnt-find-tag-with-id-for-person-with-id/19863805#comment73485730_19863805

Not sure why you wrote the gist: did the test pass (in Rails 4)? Did you try agains Rails 5?