block.ex

defmodule BlogDemo.Content.Block do
  use Ecto.Schema
  import Ecto.Changeset
  alias __MODULE__

  @derive {Jason.Encoder, []}
  embedded_schema do
    field :content, :map
    field :type, :string
    field :blocks, {:array, Block}, default: []
  end

  @doc false
  def changeset(schema, params) do
    schema
    |> cast(params, [:content, :type, :blocks])
  end


  def cast(%{ "type" => type, "content" => content, "blocks" => blocks}) do
    {:ok, %{
      type: type,
      content: content,
      blocks: Enum.map(blocks, &Block.cast/1)
    }}
  end
end

post.ex

defmodule BlogDemo.Content.Post do
  use Ecto.Schema
  import Ecto.Changeset
  alias BlogDemo.Content.Block

  schema "posts" do
    field :body, :string
    field :title, :string

    embeds_many :blocks, Block, on_replace: :delete

    timestamps()
  end

  @doc false
  def changeset(post, attrs) do
    post
    |> cast(decode_blocks(attrs), [:body, :title]) # cast_embed below fails if `attrs.blocks` is not already parsed as a struct
    |> cast_embed(:blocks)
    |> validate_required([:body, :title])
  end

  def decode_blocks(%{ "blocks" => blocks } = attrs) when is_binary(blocks) do
    Map.put(attrs, "blocks", Jason.decode!(blocks))
  end

  def decode_blocks(attrs), do: attrs
end

post_controller.ex

# ..

  def edit(conn, %{"id" => id}) do
    post = id
      |> Content.get_post!()
      |> Content.encode_blocks() # encode blocks map to string, to insert into text input value attribute

    changeset = Content.change_post(post)
    render(conn, "edit.html", post: post, changeset: changeset)
  end
  
# ..