classとメソッドの設計は適当ですが、こんなイメージです。

example2.rb

count = 0

if target_user_ids.empty?
  count += Foo.all.count
else
  target_user_ids.each do |user_ids|
    count += Foo.where(user_id: user_ids).count
  end
end

foo.rb

class Foo
  def func(arg1, arg2)
    return arg1 if arg2.empty?

    arg2.each_slice(2).map do |e2|
      arg1 & e2
    end.flatten
  end
end

foo_spec.rb

require 'spec_helper'

describe 'Foo' do
  let(:foo) { Foo.new }

  context 'with empty array' do
    let(:ary1) { [4,3,2,1] }
    let(:ary2) { [] }
    it 'returns whole ary1' do
      expect(foo.func(ary1, ary2)).to eq(ary1)
    end
  end

  context 'without empty array' do
    let(:ary1) { [6,4,3,1] }
    let(:ary2) { [1,2,4] }
    it 'returns ary1 slected ary2' do
      expect(foo.func(ary1, ary2)).to eq([1,4])
    end
  end
end