[Rails] Math between Models

I have two models that are associated with has_and_belongs_to_many, and
I'm trying to create a method that computes something with attributes
from both models, but I'm getting an undefined method error with the
code I'm using below. What am I missing?

Model_1.rb

Class Model_1 < ActiveRecord::Base
...
has_and_belongs_to_many :model_2

def some_method
attr_1 * Model_2.find(model_2_id).attr_I_need
end

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