Probably something like this…

  @doc false
  def changeset(whatever, attrs) do
    whatever
    |> changeset(attrs)
    |> cast(attrs, @fields)
    |> validate ...
    |> fingerprint()
    |> unique_constraint(:fingerprint, message: "Fingerprint already in use")
  end

  defp fingerprint(changeset) do
    case changeset do
      %Ecto.Changeset{valid?: true, changes: %{address: address}} ->
        put_change(changeset, :fingerprint, create_fingerprint(address))
      _ ->
        changeset
    end
  end

  defp create_fingerprint(address) do
    # Implement your fingerprint here
  end

It is not tested, and fields might not correspond to your structure, but You can get the idea… the 2 last lines from changeset are important.

This should not work… as it should be a field

You should use

%Ecto.Changeset{valid?: true, changes: %{street1: street1, street2: street2...}}

# or

%Ecto.Changeset{valid?: true, changes: address}

In the example I gave, whatever is the shemas, and address is just a field of %Whatever{}

You might find ecto changeset structure here. From docs…

changes - The changes from parameters that were approved in casting

You might show your schema, and migration file. Be sure :fingerprint field exists, as string…

You might also insert some IO.inspect in your pipeline to show the changeset, like so

  def changeset(whatever, attrs) do
    whatever
    |> changeset(attrs)
    |> cast(attrs, @fields)
    |> validate ...
    |> IO.inspect()
    |> fingerprint()
    |> IO.inspect()
    |> unique_constraint(:fingerprint, message: "Fingerprint already in use")
  end

The function fingerprint take a changeset, and if it is valid, it should persists the change.

If it is clearer, You could have used pattern matching, like so.

defp fingerprint(%Ecto.Changeset{valid?: true, changes: %{address: address}} = changeset) do
  put_change(changeset, :fingerprint, create_fingerprint(address))
end
defp fingerprint(changeset), do: changeset

BTW Try to change the name of the function to generate_fingerprint(), just to be sure it does not conflict with the field name.

Do not cast :fingerprint… it will be calculated

I’d also recommend you store the fingerprint in the database and add a unique constraint and a non-null constraint on the column.