Ecto Query - Dates + Postgres Intervals + Query Interpolation

Question

I would like to create an Ecto query that filters records in a children table by their age (i.e. "minimum age (months) -> maximum age (months)".

One simple way to do this would be the Ecto date_add feature:

from c in Child, where: c.birthday >
                          datetime_add(^Ecto.DateTime.utc, -1, "month")

The issue with this is that not all children will be on the same time zone, and certainly not all on Etc/UTC. This query would be pretty close, but not spot on (some would be off by a day).

I've been trying to use PostgreSQL's interval functionality to make this query work. I can get it to work using an SQL client, but I'm getting interpolation issues when trying to interpolate values in a fragment.

This works (a child's time zone is sourced from it's location association):

query = from ch in Child, 
            join: loc in assoc(ch, :location),
            where: ch.birthday <= fragment("(now() AT TIME ZONE ?)::date - interval '2 months'", loc.time_zone)

Repo.all(query)

Note that I've hard-coded in the '2 months' interval.

I thought this would work, but does not:

query = from ch in Child, 
            join: loc in assoc(ch, :location),
            where: ch.birthday <= fragment("(now() AT TIME ZONE ?)::date - interval ?", loc.time_zone, ^"2 months")

Repo.all(query)

Note that I'm trying to use Ecto's query interpolation to bring in the '2 months' value into the query.

The error is as follows:

[debug] QUERY ERROR source="children" db=1.7ms queue=0.1ms
SELECT c0."id", (... other properties) FROM "children" AS c0 INNER JOIN "programs" AS p2 ON p2."id" = c0."program_id" INNER JOIN "locations" AS l1 ON l1."id" = p2."location_id" WHERE (c0."birthday" <= (now() AT TIME ZONE l1."time_zone")::date - interval $1) ["2 months"]
** (Postgrex.Error) ERROR 42601 (syntax_error): syntax error at or near "$1"
    (ecto) lib/ecto/adapters/sql.ex:436: Ecto.Adapters.SQL.execute_and_cache/7
    (ecto) lib/ecto/repo/queryable.ex:130: Ecto.Repo.Queryable.execute/5
    (ecto) lib/ecto/repo/queryable.ex:35: Ecto.Repo.Queryable.all/4

The part of the query that fails (I tried the same query in an SQL client) is:

(now() AT TIME ZONE l1."time_zone")::date - interval $1)

It doesn't like the $1 part right there. Is it impossible to interpolate values into this kind of query?

I tried using single-quotes in an SQL client, but got the same errors. I tried the following:

SELECT c0."id" FROM "children" AS c0 INNER JOIN "programs" AS p2 ON p2."id" = c0."program_id" INNER JOIN "locations" AS l1 ON l1."id" = p2."location_id" WHERE (c0."birthday" <= (now() AT TIME ZONE l1."time_zone")::date - interval $1) ['2 months']

Any help would be appreciated!

Answer 1

I needed to do exactly this a while ago and ended up using the fact that you can multiply intervals with $1.

postgres=# select interval '1 year' - interval '1 month' * 5;
 ?column?
----------
 7 mons
(1 row)

So, this should work:

query = from ch in Child, 
            join: loc in assoc(ch, :location),
            where: ch.birthday <= fragment("(now() AT TIME ZONE ?)::date - interval '1 month' * ?", loc.time_zone, 2)

Repo.all(query)