habtm.rb

class A < ActiveRecord::Base
  has_and_belongs_to_many :bs
end

class B < ActiveRecord::Base
  has_and_belongs_to_many :as
end

class Ab < ActiveRecord::Base
  self.table_name = :as_bs
end

RSpec.describe A, type: :model do
  let(:n) { 10 }

  it do
    a = A.create
    a.bs = n.times.map { B.create }
    a.save

    expect(a.bs.count).to eq(n)
    expect(B.count).to eq(n)
    expect(Ab.count).to eq(n)

    expect do
      expect do
        a.bs.destroy_all
      end.not_to change(B, :count)
    end.to change(Ab, :count).by(-n)
  end
end