Occasionally, when I run this code in a multi-threaded environment, I observe the provided error when accessing for example apple.tags.

output.txt

$ ./tags.rb
apple
edible

stacktrace.txt

undefined method `push' for nil:NilClass
/usr/java/jruby-1.7.4/lib/ruby/gems/shared/gems/sequel-4.7.0/lib/sequel/model/associations.rb:1231:in `def_many_to_many'
org/jruby/RubyArray.java:1617:in `each'
/usr/java/jruby-1.7.4/lib/ruby/gems/shared/gems/sequel-4.7.0/lib/sequel/model/associations.rb:1231:in `def_many_to_many'
org/jruby/RubyArray.java:1617:in `each'
/usr/java/jruby-1.7.4/lib/ruby/gems/shared/gems/sequel-4.7.0/lib/sequel/dataset/actions.rb:48:in `all'
/usr/java/jruby-1.7.4/lib/ruby/gems/shared/gems/sequel-4.7.0/lib/sequel/plugins/eager_each.rb:41:in `all'
/usr/java/jruby-1.7.4/lib/ruby/gems/shared/gems/sequel-4.7.0/lib/sequel/model/associations.rb:1223:in `def_many_to_many'
org/jruby/RubyProc.java:255:in `call'
/usr/java/jruby-1.7.4/lib/ruby/gems/shared/gems/sequel-4.7.0/lib/sequel/model/associations.rb:2187:in `eager_load'
org/jruby/RubyArray.java:1617:in `each'
/usr/java/jruby-1.7.4/lib/ruby/gems/shared/gems/sequel-4.7.0/lib/sequel/model/associations.rb:2151:in `eager_load'
org/jruby/RubyBasicObject.java:1715:in `__send__'
org/jruby/RubyKernel.java:2217:in `send'
/usr/java/jruby-1.7.4/lib/ruby/gems/shared/gems/sequel-4.7.0/lib/sequel/plugins/tactical_eager_loading.rb:47:in `load_associated_objects'
/usr/java/jruby-1.7.4/lib/ruby/gems/shared/gems/sequel-4.7.0/lib/sequel/model/associations.rb:1179:in `tags'
/tmp/tags.rb:80:in `tags'

tags.rb

#! /usr/bin/env jruby

require 'sequel'

`psql -U postgres -c 'DROP DATABASE test'`
`psql -U postgres -c 'CREATE DATABASE test'`
`psql -U postgres -c "DROP USER test"`
`psql -U postgres -c "CREATE USER test PASSWORD 'test'"`

Sequel.connect 'jdbc:postgresql://localhost/test?user=test&password=test'
Sequel::Model.plugin :tactical_eager_loading
Sequel::Model.plugin :eager_each

Sequel::Model.db.create_table? :objects do
  primary_key :id
  index :name
  String :name
  String :object_type, :null => false
end

Sequel::Model.db.create_table! :tags do
  primary_key :id
  index :name
  String  :name, :unique => true, :null => false
end

Sequel::Model.db.create_table! :tagged do
  primary_key :id
  foreign_key :object_id, :objects, :on_delete => :cascade
  foreign_key :tag_id, :tags, :on_delete => :cascade
end

module Named
  def to_s; self.name; end
end

class Tag < Sequel::Model
  include Named
end

class Tagged < Sequel::Model
end

module Taggable
  def tag(*args)
    a = args.flatten.collect!(&:to_s)
    for tag in a - (tags & a)
      add_tag Tag.find_or_create(:name => tag)
    end
    save
  end

  def untag(*args)
    for tag in tags & args.flatten.collect!(&:to_s)
      remove_tag Tag.find(:name => tag)
    end
    save
  end
end

module Z
class Object < Sequel::Model
  include Named
  include Taggable
  plugin :single_table_inheritance, :object_type
  plugin :tactical_eager_loading
  one_to_many :tagged, :key => :object_id
  many_to_many :tags, :class => 'Tag', :join_table => :tagged, :left_key => :object_id, :right_key => :tag_id
end
end

apple = Z::Object.create(:name => 'apple')
apple.tag :edible

puts apple
puts apple.tags

Sequel::Model.db.disconnect

`psql -U postgres -c 'DROP DATABASE test'`
`psql -U postgres -c "DROP USER test"`