def png
      if self.png_id.nil?
       png = obconversion(smiles,"smi","_png2")
       file = Mongo::Grid::File.new(Base64.encode64(png), :filename => "#{id}.png", :content_type => "image/png")
       update(:png_id => $gridfs.insert_one(file))
      end
      Base64.decode64($gridfs.find_one(_id: self.png_id).data)
    end
def png
      if self.png_id.nil?
       png = obconversion(smiles,"smi","_png2")
       file = Mongo::Grid::File.new(Base64.encode64(png), :filename => "#{id}.png", :content_type => "image/png")
       update(:png_id => $gridfs.insert_one(file))
      end
      Base64.decode64($gridfs.find_one(_id: self.png_id).data)
    end
Get png image
 @example
   image = compound.png
 @return [image/png] Image data