Bug with multiple databases, through association, and after_commit callback

multi-db-after-commit-bug.rb

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  gem "rails", path: "/Users/composerinteralia/code/rails/rails"
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"

db = Tempfile.new(['db', '.db'])
db2 = Tempfile.new(['db2', '.db'])
ActiveRecord::Base.configurations = { issues: { adapter: "sqlite3", database: db2 } }
ActiveRecord::Base.legacy_connection_handling = false
ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: db)
ActiveRecord::Base.establish_connection(:issues)
ActiveRecord::Base.logger = Logger.new(STDOUT)

class Issues < ActiveRecord::Base
  self.abstract_class = true

  # The test passes if we comment this out so everything is in one database
  connects_to database: { writing: :issues, reading: :issues }
end

ActiveRecord::Schema.define do
  create_table :assignees, force: true do |t|
  end
end

Issues.connection.execute('CREATE TABLE "issues" ("id" integer PRIMARY KEY AUTOINCREMENT NOT NULL)')
Issues.connection.execute(<<~SQL)
  CREATE TABLE "assignments" (
    "id" integer PRIMARY KEY AUTOINCREMENT NOT NULL,
    "issue_id" integer,
    "assignee_id" integer
  )
SQL

class Issue < Issues
  # The test also passes if we set `inverse_of: false` here since we'll end up with a new issue object and load the assignees on that instead
  has_many :assignments
  has_many :assignees, through: :assignments, disable_joins: true
end

class Assignment < Issues
  belongs_to :issue
  belongs_to :assignee

  after_commit :load_issue_assignees

  def load_issue_assignees
    issue.assignees.to_a
  end
end

class Assignee < ActiveRecord::Base
  has_many :assignments
end

class BugTest < Minitest::Test
  def test_association_stuff
    issue = Issue.create!
    user = Assignee.create

    # When the models are in different databases, the after commit runs earlier
    # (presumably because a transaction from one database won't be joinable from
    # another) and so the `assignees` association gets loaded, but then we add
    # this new assignee to the association target even though it's already there
    # at https://github.com/rails/rails/blob/ced387ee58e9fd980217dd1f472f2c5afa241e9a/activerecord/lib/active_record/associations/collection_association.rb#L467-L470
    #
    # When the models are in the same database, this happens in the opposite
    # order because the after_commit runs later. We add the new assignee to
    # the association target, then later the after_commit runs and loads the
    # whole association.
    issue.assignees << user

    assert_equal [user], issue.assignees.to_a
    # expected: [#<Assignee id: 1>]
    # actual: [#<Assignee id: 1>, #<Assignee id: 1>]
  end
end