Representing complex associations in Ecto

create_locations.exs

defmodule TaskRunner.Repo.Migrations.CreateLocations do
  use Ecto.Migration

  def change do
    create table(:locations) do
      add :type, :string
    end
  end
end

create_task.exs

defmodule TaskRunner.Repo.Migrations.CreateTasks do
  use Ecto.Migration

  def change do
    create table(:tasks) do
      add :name, :string
      add :source_id, references(:locations)
      add :destination_id, references(:locations)
      add :schedule, :string
    end
  end
end

location.ex

defmodule TaskRunner.Models.Location do
  use Ecto.Schema
  import Ecto.Changeset
  alias TaskRunner.Models.Task

  schema "locations" do
    field :type, :string
    belongs_to :source, Task
    belongs_to :destination, Task
  end

  @required_fields ~w(type)
  @optional_fields ~w()

  def changeset(location, params \\ :empty) do
    location
    |> cast(params, @required_fields, @optional_fields)
  end
end

task.ex

defmodule TaskRunner.Models.Task do
  use Ecto.Schema
  import Ecto.Changeset

  alias TaskRunner.Models.Location

  schema "tasks" do
    field :name, :string
    field :schedule, :string
    # has_one :source, Location
    # has_one :destination, Location
  end

  @required_fields ~w(name schedule)
  @optional_fields ~w()

  def changeset(task, params \\ :empty) do
    task
    |> cast(params, @required_fields, @optional_fields)
  end
end