Creating Records via #new on an AR::Relation object can fail due to enums.

rails_enum_repro.rb

begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  gem 'rails', '4.2.3' # github: 'rails/rails'
  # gem 'arel', github: 'rails/arel'
  gem 'sqlite3'
end

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 :things, force: true  do |t|
    t.integer :enum_field
  end
end



class Thing < ActiveRecord::Base
  enum enum_field: {
    foo: 1,
    bar: 2,
    baz: 3
  }
end

class BugTest < Minitest::Test
  def test_creating_new_record_from_existing_relation
    Thing.all.new # OK
    Thing.all.where(enum_field: 2).new # OK
  end

  def test_creating_new_record_from_existing_relation_with_single_element_in_array
    Thing.all.where(enum_field: [1]).new # OK, since it gets coerced into the same as `enum_field: 1`
  end

  def test_creating_new_record_from_existing_relation_with_multiple_elements_in_array
    Thing.all.where(enum_field: [1, 2]).new # FAIL, despite being the valid way to craft IN() statements
  end
end