Fields

_fields.html.erb

<fieldset>
  <div class="form-inline" style="margin-bottom:10px;">

    <div class="form-group">
      <%= f.text_field "synonyms[value]", class: "form-control" %>
    </div>

    <div class="form-group">
      <%= f.text_field "synonyms[synonyms]", class: "form-control", style: 'width: 300px;' %>
    </div>
    
    <%= f.hidden_field :destroy %>
    
    <button class="btn btn-default remove_fields">X</button>
  </div>
</fieldset>

_form.html.erb

    <%= form_for @entity, url: entity_path, method: type_metod do |f| %>
      
        <%= f.fields_for :synonyms do |fields|%>

          <% @entity.synonyms.each do |key| %>
            <%= render partial: 'fields', locals: {f: fields} %>
          <% end %>

            <%= link_to_add_fields "Adicionar", f, :fields %>
        <% end %>
    <% end %>

entity.js

  $(document).on('click', 'form .remove_fields', function (event) {
      $(this).prev('input[type=hidden]').val('1');
      $(this).closest('fieldset').hide();
      return event.preventDefault();
    });

    $(document).on('click', 'form .add_fields', function (event) {
      var regexp, time;
      time = new Date().getTime();
      regexp = new RegExp($(this).data('id'), 'g');
      $(this).before($(this).data('fields').replace(regexp, time));
      return event.preventDefault();
    });

entity.rb

class Entity
  include ActiveModel::Validations
  include ActiveModel::Serialization
  include ActiveModel::Conversion
  include ActiveModel::Model
  extend ActiveModel::Naming

  attr_accessor :id, :name, :synonyms, :description
  
  ...
  

Parameters: {"utf8"=>"✓", "authenticity_token"=>"XXX==", "entity"=>{"name"=>"", "description"=>"", "synonyms"=>{"synonyms"=>{"value"=>"v1", "synonyms"=>"sinonimo1, sinonimo2"}, "destroy"=>""}}, "commit"=>"Salvar"}