I could do that like this:

     update_args = Keyword.new([{f, new}])

     from(
       a in table,
       update: [set: ^update_args],
       where: field(a, ^f) == ^old
     )
     |> MyApp.Repo.update_all([])

Wonderful! Thank you very much for solving this problem.

For people, like me, who need this to be sort-of generalized, the following MOSTLY works. It turns out that I needed to do {:replace, columns} rather than :replace_all_except_primary_key because my input data was not including that, so there’s some behind the scenes stuff coming in Ecto 3.0 which will make this very clean, but this will generalize this in the short term.

defmodule ConflictUpdate do
  @moduledoc """
  A function to create an update query for an `on_conflict:` clause in
  `insert_all`.
  """

  @doc "A function to create an update query for a schema."
  @spec conflict_update(Ecto.Schema.t(), :replace_all | :replace_all_except_primary_key |
                               {:replace, list(atom)}) :: Macro.t()
  defmacro conflict_update(schema, pattern) do
    e_schema = Macro.expand_once(schema, __CALLER__)
    e_pattern = Macro.expand_once(pattern, __CALLER__)

    update_args =
      e_schema
      |> columns(e_pattern)
      |> Enum.map(& {&1, {:fragment, [], ["EXCLUDED.#{&1}"]}})
      |> Keyword.new

    {:update, [context: Elixir, import: Ecto.Query],
      [
        schema,
        [
          set: update_args
        ]
      ]
    }
  end

  defp columns(schema, :replace_all), do: schema.__schema__(:fields)

  defp columns(schema, :replace_all_except_primary_key),
  do: schema.__schema__(:fields) -- schema.__schema__(:primary_key)

  defp columns(_schema, {:replace, fields}), do: fields
end