Inserting an model into a many-to-many relationship with an existing model in ecto 2 - phoenix-framework

Question

I'm trying out the Ecto 2 rc.
My models are:
schema "containers" do
field :name, :string
many_to_many :items, Test.Item, join_through: Test.ContainerItem, on_delete: :delete_all
timestamps
end
schema "items" do
field :content, :string
many_to_many :containers, Test.Container, join_through: Test.ContainerItem, on_delete: :delete_all
timestamps
end
schema "containers_items" do
belongs_to :container, Test.Container
belongs_to :item, Test.Item
timestamps
end
And my controller code is:
def add_item(conn, %{"item" => item_params, "container_id" => container_id}) do
item = Item.changeset(%Item{}, item_params)
IO.inspect(item) #TODO remove
container = Container |> Repo.get(container_id) |> Repo.preload([:items])
IO.inspect(container) #TODO remove
changeset = container
|> Ecto.Changeset.change
|> Ecto.Changeset.put_assoc(:items, [item])
IO.inspect(changeset) #TODO remove
if changeset.valid? do
Repo.update(changeset)
conn
|> put_flash(:info, "Item added.")
|> redirect(to: container_path(conn, :show, container))
else
render(conn, "show.html", container: container, changeset: changeset)
end
end
Now this works fine if I'm adding a single item to a container. However if an item exists on a container, then trying to add another item gives me:
(RuntimeError) you are attempting to change relation :items of
Test.Container, but there is missing data.
I can't help but feel I'm going about this the wrong way, some advice would be appreciated.

Answer 1

Ok, so I just figured this out.
My problem was is not turning the items into Changesets so that ecto can track the changes that it needs to make.
The only edits I needed to make are to the controller.
It should look like this instead:
def add_item(conn, %{"item" => item_params, "container_id" => container_id}) do
item = Item.changeset(%Item{}, item_params)
IO.inspect(item) #TODO remove
container = Container |> Repo.get(container_id) |> Repo.preload([:items])
IO.inspect(container) #TODO remove
item_changesets = Enum.map([item | container.items], &Ecto.Changeset.change/1)
changeset = container
|> Ecto.Changeset.change
|> Ecto.Changeset.put_assoc(:items, item_changesets)
IO.inspect(changeset) #TODO remove
if changeset.valid? do
Repo.update(changeset)
conn
|> put_flash(:info, "Item added.")
|> redirect(to: container_path(conn, :show, container))
else
render(conn, "show.html", container: container, changeset: changeset)
end
end