Dynamically add/remove nested fields to your Phoenix forms from the client with a thin JavaScript layer.

1. The package can be installed by adding dynamic_inputs_for to your list of dependencies in mix.exs:

def deps do
  [{:dynamic_inputs_for, "~> 1.1.0"}]
end

2. Then add dynamic_inputs_for to your list of dependencies in package.json and run npm install. For the default Phoenix structure, in assets/package.json:

"dependencies": {
  "dynamic_inputs_for": "file:../deps/dynamic_inputs_for"
}

3. Finally, don't forget to import the module. For the default Phoenix structure, in assets/js/app.js:

import "dynamic_inputs_for";

Imagine the following Ecto schemas:

defmodule Shop do
  use Ecto.Schema

  schema "shops" do
    field :name, :string
    has_many :products, Product
  end
end

defmodule Product do
  use Ecto.Schema

  schema "products" do
    field :name, :string
    ...

    belongs_to(:shop, Shop)
  end
end

If you want to be able to dynamically add products in a form, use the dynamic_inputs_for helper in combination with dynamic_add_button to generate the form.

If you also want to allow the deletion of nested fields, this library follows the strategy suggested in the Ecto.Changeset documentation. Add a separate boolean virtual field to the changeset function that will allow you to manually mark the associated data for deletion and use the dynamic_delete_button helper inside the function that you pass to dynamic_inputs_for to generate a delete button for each associated data.

defmodule Product do
  use Ecto.Schema
  import Ecto.Changeset

  schema "products" do
    field :name, :string
    ...
    field :delete, :boolean, virtual: true

    belongs_to(:shop, Shop)
  end

  def changeset(product, params) do
    product
    |> cast(params, [:name, :delete])
    |> maybe_mark_for_deletion
  end

  defp maybe_mark_for_deletion(changeset) do
    if get_change(changeset, :delete) do
      %{changeset | action: :delete}
    else
      changeset
    end
  end
end

<%= form_for @changeset, Routes.shop_path(@conn, :create), fn f -> %>
  <%= text_input f, :name %>

  <%= dynamic_inputs_for f, :products, %Product{}, fn f_product -> %>
    <%= text_input f_product, :name %>

    <%= dynamic_delete_button("Delete") %>
  <% end%>

  <%= dynamic_add_button :products, "Add" %>
<% end %>

If you want the new fields to have default values, you can pass them to the schema you pass to dynamic_inputs_for. In the previous example %Product{name: "ASDF"}.

<%= dynamic_inputs_for f, :products, %Product{name: "ASDF"}, fn f_product -> %>

When you add or delete an element, the events dynamic:addedFields and dynamic:deletedFields are triggered. These events can be listened to modify the nested fields or integrate them with third party javascript libraries.

document.addEventListener(
  "dynamic:addedFields",
  function(e) {
    e.target.style.backgroundColor = "red";
  },
  false
);

or if you use jQuery

$(document).on("dynamic:addedFields", e => {
  e.target.style.backgroundColor = "red";
});