问将小时添加到日期时间字段ecto elixir

EN

schema "fixtures" do
    field :sm_id, :integer
    field :local_score, :integer
    field :visitor_score, :integer
    field :local_pen_score, :integer
    field :visitor_pen_score, :integer
    field :ht_score, :string
    field :ft_score, :string
    field :et_score, :string
    field :starting_at, Ecto.DateTime
    belongs_to :local_team, Team, foreign_key: :local_team_id
    belongs_to :visitor_team, Team, foreign_key: :visitor_team_id
    belongs_to :season, Season
    belongs_to :round, Round

    timestamps()
end

def fixtures_live(query, round_) do
    now = Ecto.DateTime.utc |> Ecto.DateTime.cast!
    query
    |> join(:left, [r], f in assoc(r, :fixtures))
    |> where([r, _], r.id == ^round_.id)
    |> where([_, f], f.starting_at < ^now)
    |> where([_, f], datetime_add(f.starting_at, 2, "hour") > ^now)
    |> select([_, f], f)
end

Ecto.Query.CastError at GET /sports/get_all_fixtures
web/models/round.ex:73: value `#Ecto.DateTime<2017-08-02 16:32:29>` in `where` cannot be cast to type :naive_datetime in query:

    |> where([_, f], datetime_add(f.starting_at, 2, "hour") |> Ecto.DateTime.cast! > ^now)

Compiling 11 files (.ex)

== Compilation error in file web/models/round.ex ==
** (Ecto.Query.CompileError) `Ecto.DateTime.cast!(datetime_add(f.starting_at(), 2, "hour"))` is not a valid query expression
    (ecto) expanding macro: Ecto.Query.where/3
    (sopitas) web/models/round.ex:73: Sopitas.Round.fixtures_live/2
    (ecto) expanding macro: Ecto.Query.select/3
    (sopitas) web/models/round.ex:74: Sopitas.Round.fixtures_live/2
    (elixir) expanding macro: Kernel.|>/2
    (sopitas) web/models/round.ex:74: Sopitas.Round.fixtures_live/2
    (elixir) lib/kernel/parallel_compiler.ex:121: anonymous fn/4 in Kernel.ParallelCompiler.spawn_compilers/1