[Rails] Ensuring that foreign key is valid

I am trying to set up a validation in a model to ensure that a record
can't be saved unless the foreign key it belongs_to is a valid record.
My model says:

class Admin < ActiveRecord::Base
validates_presence_of :user_id
validates_numericality_of :user_id
validates_uniqueness_of :user_id
validates_associated :user

belongs_to :user

end

but I can still create an admin record even if the associated user
record doesn't exist. I tried creating a custom validator, too, but I
couldn't test User.find(@user_id) from within the model.

How can I check for this at the model level?

--
"Oh, look: rocks!"
-- Doctor Who, "Destiny of the Daleks"

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