medium_virtual_example.ex

defmodule BlogExample.Schema.PostWithVirtualCategory do
  use Ecto.Schema

  schema "posts" do
    field(:title, :string, null: false)
    field(:contents, :string, null: false)
    field(:category_name, :string)

    field(:category, :map, virtual: true)

    belongs_to(:user, BlogExample.Schema.User)
  end

  def populate_category(%__MODULE__{category_name: nil} = post) do
    post
  end

  def populate_category(%__MODULE__{category_name: category_name} = post) do
    category = BlogExample.RemoteService.retrieve_category_info(category_name)
    %__MODULE__{post | category: category}
  end
end