Express a CTE using Arel

I have this snippet of ActiveRecord:

scope = Listing.where(Listing.arel_table[:price].gt(6_000_000))

The resulting sql:

SELECT listings.* FROM listings where listings.price > 6000000

I would like to add a CTE that would result in this sql:

WITH lookup AS (
    SELECT the_geom FROM lookup WHERE slug = 'foo-bar'
)

SELECT * from listings, lookup
WHERE listings.price > 6000000
AND ST_within(listings.the_geom, lookup.the_geom)

I would like to express this sql inclusive of the CTE using Arel and ActiveRecord.

I would also like to use the scope variable as a starting point.