missing binds

arel_example.rb

gem 'activerecord', '4.1.10'#uncomment to load latest AR gem
require 'active_record'

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

puts "-- arel #{Arel::VERSION}"

ActiveRecord::Migration.verbose = false
ActiveRecord::Schema.define do
  create_table :tasks do |t|
    t.integer :assigned_id
    t.string  :assigned_type
  end
  create_table :people do |t|
    t.string :name
  end
  create_table :roles do |t|
    t.integer :person_id
  end
end

Task = Class.new ActiveRecord::Base
Role = Class.new ActiveRecord::Base

class Person < ActiveRecord::Base
  def tasks
    str = Arel::VERSION.to_i == 5 ? 'Role' : Arel::Nodes::Quoted.new('Role')
    person_arel = Task.where(assigned_id: self.id, assigned_type: 'Person')
    _roles = Role.where(person_id: self.id).select(:id).as('_roles')
    roles_arel = Task.joins(Arel::Nodes::InnerJoin.new(_roles, Arel::Nodes::On.new(
      Arel::Nodes::Equality.new(_roles[:id], Task.arel_table[:assigned_id]).and(
      Arel::Nodes::Equality.new(Task.arel_table[:assigned_type], str))
    )))
    union = person_arel.union roles_arel
    tasks = Arel::Nodes::TableAlias.new(union, :tasks)
    Task.from(tasks)
  end
end

me = Person.create(name: 'me')
puts me.tasks.to_sql

__END__
produces
-- arel 5.0.1
SELECT "tasks".* FROM (
  SELECT "tasks".* FROM "tasks"  WHERE "tasks"."assigned_id" = 1 AND "tasks"."assigned_type" = 'Person'
  UNION
  SELECT "tasks".* FROM "tasks" INNER JOIN (
    SELECT "roles"."id" FROM "roles"  WHERE "roles"."person_id" = 1
  ) _roles ON _roles."id" = "tasks"."assigned_id" AND "tasks"."assigned_type" = 'Role'
) "df"