BelongsToMany: can't reset column to empty array with defaults

Created by: yosiat

Hi,

Checked the latest commit, following the discussion here - https://github.com/crashtech/torque-postgresql/issues/56#issuecomment-777657238

# frozen_string_literal: true
require 'torque-postgresql'

require 'byebug'
require "active_record"
require "minitest/autorun"
require "logger"

ActiveRecord::Base.establish_connection(
  adapter:  "postgresql",
  database: "test",
  encoding: "unicode",
  host:     "localhost",
  port:     "5432",
  password: "12345",
  username: "test")

ActiveRecord::Schema.define do
  drop_table "employees", if_exists: true
  drop_table "projects", if_exists: true

  create_table "employees" do |t|
    t.string "name"
    t.timestamps
  end

  create_table "projects" do |t|
    t.string "title"
    t.timestamps
  end

  execute "alter table projects add column employees_ids integer[] default '{}'::integer[]"
end

class Employee < ActiveRecord::Base
  has_many :projects, array: true, foreign_key: :employees_ids
end

class Project < ActiveRecord::Base
  belongs_to_many :employees, foreign_key: "employees_ids"
end

class BugTest < Minitest::Test
  def test_bug
    employee = Employee.create!

    project = Project.create!(employees_ids: [employee.id])

    # Accessing `project.employees`, cause the next update to get dismissed
    pp project.employees

    project.update(employees_ids: [])

    project.reload

    pp project.employees_ids
    pp project.employees
  end
end

I am trying to update the employee_ids to be an empty array, and it doesn't work. This happens because ids_reader takes the ids from the target (while the target is stale)