has one of many

i just wrote a cool module, should make it into a plugin, if i had time.

has_one_of_many  lets you make a has_many association look like a has_one at runtime. In my app, X has_many Y but each user can only make one Y per X. I wanted to push the association find down into the model but normally models dont know about current_user. There's other uses as well, could be quite handy any time an action is attending to a single, currently selected item in a collection.

So, for example, if Article has many reviews, but each user can write only one review, now you can say @article.review and get the current user's one.

In the Article model you'd declare

    has_one_of_many :reviews, :by => :user_id

Then in your controller, you could say Article.current_review_user_id = current _user.id

Internally it does a self.reviews.find_by_user_id 

Possible extensions:

• has_some_of_many to return a filtered collection. The problem is i dont like the idea of replacing, for example, #reviews with the filtered one, but could use alias_method_chain. Anyway, named finders can do this already.
  

• :condition => or other find options to better control the find, such as the the most recent

Note, my app also uses a home-grown version of nested_params http://rails.lighthouseapp.com/projects/8994/tickets/1202-add-attributes-writer-method-for-an-association , so I generate a review_attributes=() method too.

Some have commented this violates MVC, but I strongly disagree. First, filtering on user is just an example. The model never knows about current_user, it only knows there's an attribute value used to constrain the collection to a single item.

Here's the code:

  # ------------------------------------

  # has_one_of_many

  # ------------------------------------

  # eg  has_one_of_many :reviews, :by => :user_id

  # implements class methods:

  #   current_review_user_id=( value ) # sets the value used to query the has_many

  #   current_review_user_id # get the value used

  # instance methods:

  #   review # => the current child, as if it were has_one
  #   review_attributes=( attribs ) # sets attributes in the current, or new if none

    @@has_current = {} # :assoc => :attrib

    @@current_value = {} # :assoc => value

    # names of associations that has current

    def self.current_ones

      @@has_current.keys

    end 

    #    

    def self.has_one_of_many(association_id, options={}) # public

      names = association_id.to_s
      name = name.singularize

      by = options[:by]

      @@has_current[name] = by

      self.class_eval %{

        def self.current_#{name}_#{by}=(value)

          @@current_value['#{name}'] = value

        end

        def self.current_#{name}_#{by}

          @@current_value['#{name}']

        end

        # instance methods

        def #{name}

          #{namepl}.find_by_#{by}( @@current_value['#{name}'] )

        end

        def #{name}_attributes=(new_attributes)

          record = self.#{names}.find_by_#{by}( @@current_value['#{name}'] )

          if record

            record.update_attributes(new_attributes)

          else

            self.#{names}.new(new_attributes)

          end

        end

      }, __FILE__, __LINE__  

    end

    # kludgey but handles :include => :review

    # since the association now looks like a has_one, 

    # but since its not really an association, AR barfs

    # so just delete it from the :include

    class << self

      def find_with_current_ones_removed(*args, &block)

        current_ones.each do |name|

          args.last[:include].delete(name.to_sym)

        end if args.last[:include]

        find_without_current_ones_removed( *args, &block )

      end

      alias_method_chain :find, :current_ones_removed

    end