Ecto custom SQL function

Are there some Ecto gurus here?

I have this SQL function which works amazingly well (using Postgres 11.6)

CREATE EXTENSION IF NOT EXISTS "unaccent";

CREATE OR REPLACE FUNCTION slugify("value" TEXT)
RETURNS TEXT AS $$
  -- removes accents (diacritic signs) from a given string --
  WITH "unaccented" AS (
    SELECT unaccent("value") AS "value"
  ),
  -- lowercases the string
  "lowercase" AS (
    SELECT lower("value") AS "value"
    FROM "unaccented"
  ),
  -- remove single and double quotes
  "removed_quotes" AS (
    SELECT regexp_replace("value", '[''"]+', '', 'gi') AS "value"
    FROM "lowercase"
  ),
  -- replaces anything that's not a letter, number, hyphen('-'), or underscore('_') with a hyphen('-')
  "hyphenated" AS (
    SELECT regexp_replace("value", '[^a-z0-9\\-_]+', '-', 'gi') AS "value"
    FROM "removed_quotes"
  ),
  -- trims hyphens('-') if they exist on the head or tail of the string
  "trimmed" AS (
    SELECT regexp_replace(regexp_replace("value", '\-+$', ''), '^\-', '') AS "value"
    FROM "hyphenated"
  )
  SELECT "value" FROM "trimmed";
$$ LANGUAGE SQL STRICT IMMUTABLE;

select * from user u
where slugify(u.administrative) = 'ile-de-france';

I have no idea how I could insert it in a ecto query, like:

    User
      |> where(slugify(:administrative, ^administrative))
      |> order_by([f], f.city)
      |> Repo.all()

Can someone help me with that? My searches for “Ecto custom SQL function” did not produce any result.

Using 100% custom SQL query would be a pain because I need to chain it with other query criteria afterwards.