_form.html.slim

= form_for(@visualization) do |f|

  = f.input :title

  = f.fields_for :inputs_visualizations do |iv|
        
    = iv.input :color

    = iv.fields_for :input do |i|
      = i.input :title

  = f.button :submit, "Save"

input.rb

class Input < ActiveRecord::Base

  # Associations ------------------
  has_many :inputs_visualizations, dependent: :destroy, order: "inputs_visualizations.order ASC"
  has_many :visualizations, through: :inputs_visualizations

  # Attributes --------------------
  attr_accessible :title, :unit

end

inputs_visualization.rb

class InputsVisualization < ActiveRecord::Base

  # Associations ------------------
  belongs_to :input
  belongs_to :visualization

  # Attributes --------------------
  attr_accessible :input_id, :visualization_id, :color, :input_attributes
  accepts_nested_attributes_for :input, :reject_if => lambda { |i| i[:title].blank? }, :allow_destroy => true

end

visualization.rb

class Visualization < ActiveRecord::Base

  # Associations ------------------
  has_many :inputs_visualizations, dependent: :destroy, order: "inputs_visualizations.order ASC"
  has_many :inputs, through: :inputs_visualizations, order: "inputs_visualizations.order ASC"

  # Attributes --------------------
  attr_accessible :title, :inputs_visualizations_attributes
  accepts_nested_attributes_for :inputs_visualizations, :reject_if => lambda { |a| a[:input_id].blank? }, :allow_destroy => true

end