How can I do this in rails active record¿?

find all models that match (created_at + 100 days between this month)

Edit: Ok, Sorry for not be precise this what I'm trying to do in active record in Rails 3.0 way:

select 
    distinct p.ID 
from
    patients p
    inner join vaccines_patients vp on p.ID = vp.patient_id
    inner join vaccines v on v.ID = vp.VACCINE_ID
where
    month(vp.appliedAt + INTERVAL v.duration DAY) = month(now())

I want to get a similar query but using where in active record.

ruby-on-rails date activerecord
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