Api code for getting results from associated tables in Ruby

Question

Hi , I am new to Ruby on rails..

I am trying to write a sample ruby code for the action that i having in my api Blogs.

  def tag
    @query_string = "Blogpost.published"
    if params[:limit]
      if params[:limit].to_i.class == Fixnum
        if params[:limit].to_i > 0
          @query_string += ".limit(#{params[:limit]})"
        end
      end
    end
    @query_string += ".find_tagged_with(\"#{params[:tag]}\")"
    @blogs = eval(@query_string)
    respond_to do |format|
        format.html { redirect_to root_path }
        format.xml  { render :xml => @blogs.to_xml(to_any_options) }
        format.json { render :json => @blogs.to_json(to_any_options) }
    end
  end

The above is my action tag ..

I am having some 3 tables .. 1. Blogpost which stores all the blog details .. 2. Tags - Each blogpost have some tags associated with it . those tags are stored in the Tags table with (id,tagname) 3. Tagging which contains the (id,tag_id,taggable_id) where tag_id is the id for the tagname from Tags table and taggable_id is the id for the BLog to which this tag is associated..

I will run the link by giving

/api/blogs/tag/.xml

which returns all the blogposts linked to the tag..

I want to write Ruby code for the same.. How to write so ??

Please give me suggestions..