How to select_merge with fragment?

I have a list like this

selected_fields = [:inbox_count, :phone_number_count, :comment_count, ...] #this list contains fields of a table, and can change by the needed

group_query = 
    from(
        ps in PageStatistic,
        where: ps.id in ['1', '2']
        group_by: fragment("date(?)", ps.hour)
        select: %{
            day: fragment("date(?)", ps.hour),
        }

final_query = 
    select_fields
        |> Enum.reduce(group_query, fn field, query ->
            field_string = to_string(field)
            select_merge(query, [ps], %{^field_string => fragment("sum(?)", field(ps, ^field))})
        )

Repo.all(final_query)

but i got result like this
[
%{
    :day => ~D[2023-01-01],
    "inbox_count" => 3,
    ...
},
...
]

Is there any way make key “inbox_count” is an atom same as :day when query(i don’t want to loop the result to convert string to atom)
And is there any solution to select sum all fields that exist in select_fields when i make a query to database

P/s: i’m using Ecto v3.0