Elixir Ecto - how to group and count records by week

@doc "Count responses per week on `date_field` (atom)"
def count_by_week(query, date_field) do
    query
    |> group_by([r], (fragment("date_part('week', ?)", (field(r, ^date_field)))))
    |> select([r], [(fragment("date_part('week', ?)", (field(r, ^date_field)))), count("*")])
end

# example model is called `Response`
def fetch_data(id) do
    Response
    |> Response.with_id(id)
    |> Response.count_by_week(:inserted_at)
    |> Repo.all
    |> Enum.map(fn([weeknr,count]) -> ["Week #{weeknr}", count] end)
end