I have a products api, database is postgresql, I use ecto.

curl "localhost:4000/products?brand=AZN&limit=1&fields=ean,date,brand"
{"data":[{"brand":"AZN","date":"2019-09-01T02:00:00Z","ean":"9783423214254"}]}

Accept can be text/csv, and then

• format of date is milliseconds from epoch.
• fields MUST be in order

curl "localhost:4000/products?brand=AZN&limit=1&fields=ean,date,brand" -H accept:text/csv
9783423214254,1567303200000,AZN

I do not know how to achieve that using select and select_merge.

When I add expression to select, one at a time using select_merge:

q = from p in Product, select: %{}
q = from p in q, select_merge: map(p, [:ean])
q = from p in q, select_merge: %{date: fragment("extract(epoch from ?)*1000", p.date)}
q = from p in q, select_merge: map(p, [:brand])
Repo.to_sql(:all, q)
{"SELECT p0.\"ean\", p0.\"brand\", extract(epoch from p0.\"date\")*1000 FROM \"products\" AS p0",
 []}

Then order is not preserved: ean, brand, date. I need ean, date, brand.
How would you do that?

Cheers.