I found an excellent guide at ElixirSchool that helped me assemble the pieces.

My code now looks like this:

The tests

  test "assosiations" do
    import MyApp.WizardFixtures
    import MyApp.CamerasFixtures
    alias MyApp.Wizard.Summary
    alias MyApp.Repo
    camera1 = camera_fixture()
    camera2 = camera_fixture()

    summary = summary_fixture()
    summary = Repo.preload(summary, [:cameras])
    summary_changeset = Ecto.Changeset.change(summary)

    summary_cameras_changeset = summary_changeset |> Ecto.Changeset.put_assoc(:cameras, [camera1, camera2])

    Repo.update!(summary_cameras_changeset)
    summary = Repo.reload(summary)
    summary = Repo.preload(summary, :cameras)
    assert summary.cameras == [camera1, camera2]

  end

The code

defmodule MyApp.SummaryCamera do
  use Ecto.Schema

  schema "summaries_cameras" do
    belongs_to :summary, MyApp.Wizard.Summary
    belongs_to :camera, MyApp.Cameras.Camera

    timestamps()
  end

  def changeset(struct, params \\ %{}) do
    struct
    |> Ecto.Changeset.cast_assoc( :camera, required: true)
    |> Ecto.Changeset.cast_assoc( :summary, required: true)
  end
end

The migration:

  def change do
    create table(:summaries_cameras, primary_key: false) do
      add :summary_id, references(:summaries)
      add :camera_id, references(:cameras)
    end

    create unique_index(:summaries_cameras,
     [:summary_id, :camera_id])
  end

defmodule MyApp.Wizard.Summary do
  use Ecto.Schema
  import Ecto.Changeset

  schema "summaries" do
  
    many_to_many :cameras, MyApp.Cameras.Camera,
   join_through: "summaries_cameras"

defmodule MyApp.Wizard.Summary do
  use Ecto.Schema
  import Ecto.Changeset

  schema "summaries" do

  many_to_many :cameras, MyApp.Cameras.Camera, 
  join_through: "summaries_cameras"