[Rails] active record question

I have three models, Question, Response and User

User has many question, :through Responses

My response table has 3 main columns response, question_id and
user_id.

The use case is lets say the user can respond to 100 question, but
during a session he responds to 20. How do I find out the ones he has
not respondeded to. In this case the response table will have 20
records and I woukd like to show the user the rest of the 80 he has
not responded to.

Thanks

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