Bug in ActiveRecord where(...).create() -- see https://github.com/rails/rails/issues/16668

test.rb

require 'active_record'
require 'sqlite3'
require 'rspec'

ActiveRecord::Base.establish_connection(
  adapter: 'sqlite3',
  database: ':memory:'
)

ActiveRecord::Base.logger = Logger.new(STDOUT)

class CreateTables < ActiveRecord::Migration
  def up
    create_table :folders do |t|
      t.integer :their_folder_id
    end
    create_table :entries do |t|
      t.integer :folder_id
      t.integer :irrelevant_number
      t.integer :their_folder_entry_count
    end
  end
end

CreateTables.new.up

class Folder < ActiveRecord::Base
  has_many :entries
  belongs_to :their_folder, class_name: 'Folder'
end

class Entry < ActiveRecord::Base
  belongs_to :folder

  before_create do
    if folder.their_folder
      self.their_folder_entry_count = folder.their_folder.entries.count
    end
  end
end

describe Folder do
  before do
    @their_folder = Folder.create
    @their_entries = (1..3).map { @their_folder.entries.create }
    @my_folder = Folder.create(their_folder: @their_folder)
  end

  context 'create without relation' do
    before do
      @entry = @my_folder.entries.create(irrelevant_number: 0)
      # SQL issued to get their_folder_count:
      # => SELECT COUNT(*) FROM "entries"  WHERE "entries"."folder_id" = 1
    end

    it 'sets their_folder_entry_count to the correct number' do
      expect(@entry.their_folder_entry_count).to eq(3)
    end
  end

  context 'create with relation' do
    before do
      @entry = @my_folder.entries.where(irrelevant_number: 0).create
      # SQL issued to get their_folder_count:
      # => SELECT COUNT(*) FROM "entries"  WHERE "entries"."irrelevant_number" = 0 AND "entries"."folder_id" = 3
    end

    it 'sets their_folder_entry_count to the correct number' do
      expect(@entry.their_folder_entry_count).to eq(3)
      # this fails with:
      #  1) Folder#create sets their_folder_entry_count to the correct number
      #     Failure/Error: expect(@entry.their_folder_entry_count).to eq(3)
      #
      #       expected: 3
      #            got: 0
    end
  end
end