PostgreSQL:намерете броя на последователните дни до момента

with t as (
    SELECT distinct(uca.created_at::date) as created_at
    FROM user_challenge_activities as uca 
    INNER JOIN user_challenges as uc ON user_challenge_id = uc.ID 
    WHERE uc.user_id = #{user.id}
    )
select count(*)
from t
where t.create_at > (
    select d.d
    from generate_series('2010-01-01'::date, CURRENT_DATE, '1 day') d(d)
    left outer join t on t.created_at = d.d::date
    where t.created_at is null
    order by d.d desc
    limit 1
)