[Rails] Help selecting the correct elements in an array

I'm working on a method but the results are "the opposite" of what I
want to happen. Here's what I have in my model:

after_update :foo

def foo
self.scores.where(:standard_id => self.standard_ids).each do |
score|
score.destroy
end
end

... which actually deletes the scores I want to keep. What I want to
do is something like:

self.scores - self.scores.where(:standard_id =>
self.standard_ids).each do...

OR

a = self.scores
b = self.scores.where(:standard_id => self.standard_ids)
a - b = c
c.each do...

OR

self.scores.reject(self.scores.where(:standard_id =>
self.standard_ids)).each do...

Any help is greatly appreciated! Thanks in advance :-)

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