I have a one-to-many relation where I – say – add items to cart. Once I add one item it’s fine. When I add another I get:

errors: [id: {"has already been taken", []}]

on the second item, even if it’s not persisted. The whole changeset looks (simplified) like:

#Ecto.Changeset<
  action: :validate,
  changes: %{
    [...]
    items: [
      #Ecto.Changeset<
        action: :insert,
        changes: %{
          description: "gin",
          [...]
        },
        errors: [],
        data: #MyApp.Cart.Item<>,
        valid?: true
      >,
      #Ecto.Changeset<
        action: :insert,
        changes: %{
          description: "tonic",
          [...]
        },
        errors: [id: {"has already been taken", []}],
        data: #MyApp.Cart.Item<>,
        valid?: false
      >
    ],
  },
  errors: [],
  [...]
  valid?: false
>

I “put” the action “validate” on the parent in the changeset like:

changeset = %Cart{}
|> Cart.changeset(add_static_fields(params, socket))
|> Map.put(:action, :validate)

but apparently children have action “insert” in their respective changesets. Since I am not “inserting” the parent anyway, why do the children complain about their "id being taken"? No id's been assigned yet, or what part of my brain is not working today?