I noticed this behavior while trying to pattern match the :changes on a changeset with an embedded schema.

I have a simple changeset/2 function defined on an embedded schema for a Webhook:

defmodule Webhook do
  embedded_schema do
    field :webhook_id, :string
    field :topic, :string
  end

  def changeset(schema, changes) do
    Ecto.Changeset.cast(schema, changes, [:webhook_id, :topic])
  end
end

Which exists on a parent Account schema:

defmodule Account do
  schema "accounts" do
    field :name, :string
    embeds_many :webhooks, Webhook
  end

  def for_insert(changes) do
    %__MODULE__{}
    |> Ecto.Changeset.cast(changes, [:name])
    |> Ecto.Changeset.cast_embed(:webhooks)
  end
end

When Webhook.changeset/2 is triggered from the cast_embed/3 in Account, action: :insert is automatically added to the embedded changeset in the returned :changes. When I call Webhook.changeset/2 directly, however, it returns action: :nil.

I assume the :action is somehow being determined by cast_embed/3 then, but I’m struggling to find exactly where that happens and / or what the reasoning is?