build

# File activerecord/lib/active_record/associations.rb, line 1893
            def build(associations, parent = nil)
              parent ||= @joins.last
              case associations
                when Symbol, String
                  reflection = parent.reflections[associations.to_s.intern] or
                  raise ConfigurationError, "Association named '#{ associations }' was not found; perhaps you misspelled it?"
                  @reflections << reflection
                  @joins << build_join_association(reflection, parent)
                when Array
                  associations.each do |association|
                    build(association, parent)
                  end
                when Hash
                  associations.keys.sort{|a,b|a.to_s<=>b.to_s}.each do |name|
                    build(name, parent)
                    build(associations[name])
                  end
                else
                  raise ConfigurationError, associations.inspect
              end
            end