class ActiveRecord::Associations::ClassMethods::JoinDependency::JoinAssociation

def association_join
  return @join if @join
  aliased_table = Arel::Table.new(table_name, :as      => @aliased_table_name,
                                              :engine  => arel_engine,
                                              :columns => klass.columns)
  parent_table = Arel::Table.new(parent.table_name, :as      => parent.aliased_table_name,
                                                    :engine  => arel_engine,
                                                    :columns => parent.active_record.columns)
  as_conditions = reflection.options[:conditions] && process_conditions(reflection.options[:conditions], aliased_table_name)
  @join = case reflection.macro
  when :has_and_belongs_to_many
    join_table = Arel::Table.new(options[:join_table], :as => aliased_join_table_name, :engine => arel_engine)
    fk = options[:foreign_key] || reflection.active_record.to_s.foreign_key
    klass_fk = options[:association_foreign_key] || klass.to_s.foreign_key
    [
      join_table[fk].eq(parent_table[reflection.active_record.primary_key]),
      [aliased_table[klass.primary_key].eq(join_table[klass_fk]), as_conditions].reject{ |x| x.blank? }
    ]
  when :has_many, :has_one
    if reflection.options[:through]
      join_table = Arel::Table.new(through_reflection.klass.table_name, :as => aliased_join_table_name, :engine => arel_engine)
      jt_as_conditions = through_reflection.options[:conditions] && process_conditions(through_reflection.options[:conditions], aliased_table_name)
      jt_as_extra = jt_source_extra = jt_sti_extra = nil
      first_key = second_key = as_extra = nil
      if through_reflection.macro == :belongs_to
        jt_primary_key = through_reflection.primary_key_name
        jt_foreign_key = through_reflection.association_primary_key
      else
        jt_primary_key = through_reflection.active_record_primary_key
        jt_foreign_key = through_reflection.primary_key_name
        if through_reflection.options[:as] # has_many :through against a polymorphic join
          jt_as_extra = join_table[through_reflection.options[:as].to_s + '_type'].eq(parent.active_record.base_class.name)
        end
      end
      case source_reflection.macro
      when :has_many, :has_one
        if source_reflection.options[:as]
          first_key   = "#{source_reflection.options[:as]}_id"
          second_key  = options[:foreign_key] || primary_key
          as_extra    = aliased_table["#{source_reflection.options[:as]}_type"].eq(source_reflection.active_record.base_class.name)
        else
          first_key   = through_reflection.klass.base_class.to_s.foreign_key
          second_key  = options[:foreign_key] || primary_key
        end
        unless through_reflection.klass.descends_from_active_record?
          jt_sti_extra = join_table[through_reflection.active_record.inheritance_column].eq(through_reflection.klass.sti_name)
        end
      when :belongs_to
        first_key = primary_key
        if reflection.options[:source_type]
          second_key = source_reflection.association_foreign_key
          jt_source_extra = join_table[reflection.source_reflection.options[:foreign_type]].eq(reflection.options[:source_type])
        else
          second_key = source_reflection.primary_key_name
        end
      end
      [
        [parent_table[jt_primary_key].eq(join_table[jt_foreign_key]), jt_as_extra, jt_source_extra, jt_sti_extra, jt_as_conditions].reject{|x| x.blank? },
        [aliased_table[first_key].eq(join_table[second_key]), as_extra, as_conditions].reject{ |x| x.blank? }
      ]
    elsif reflection.options[:as]
      id_rel = aliased_table["#{reflection.options[:as]}_id"].eq(parent_table[parent.primary_key])
      type_rel = aliased_table["#{reflection.options[:as]}_type"].eq(parent.active_record.base_class.name)
      [id_rel, type_rel, as_conditions].reject{ |x| x.blank?}
    else
      foreign_key = options[:foreign_key] || reflection.active_record.name.foreign_key
      [aliased_table[foreign_key].eq(parent_table[reflection.options[:primary_key] || parent.primary_key]), as_conditions].reject{ |x| x.blank? }
    end
  when :belongs_to
    [aliased_table[options[:primary_key] || reflection.klass.primary_key].eq(parent_table[options[:foreign_key] || reflection.primary_key_name]), as_conditions].reject{ |x| x.blank? }
  end
  unless klass.descends_from_active_record?
    sti_column = aliased_table[klass.inheritance_column]
    sti_condition = sti_column.eq(klass.sti_name)
    klass.descendants.each {|subclass| sti_condition = sti_condition.or(sti_column.eq(subclass.sti_name)) }
    @join << sti_condition
  end
  @join
end