How to test composite Rails model scopes?

Consider the following model:

class Model < ActiveRecord::Base
  scope :a,       -> () { where(a: 1) }
  scope :b,       -> () { where(b: 1) }
  scope :a_or_b,  -> () { a.or(b) }
end

now to properly test each scope, I would at least provide an exaple that matches and one that doesn''t for each possible variable. Something like this:

RSpec.describe Model do
  describe "scopes" do
    describe ".a" do
      subject { Model.a }
      let!(:with_a_0) { create(:model, a: 0) }
      let!(:with_a_1) { create(:model, a: 1) }

      it { should_not include(with_a_0) }
      it { should     include(with_a_1) }
    end

    describe ".b" do
      subject { Model.b }
      let!(:with_b_0) { create(:model, b: 0) }
      let!(:with_b_1) { create(:model, b: 1) }

      it { should_not include(with_b_0) }
      it { should     include(with_b_1) }
    end

    describe ".a_or_b" do
      subject { Model.a_or_b }
      let!(:with_a_0_and_b_0) { create(:model, a: 0, b: 0) }
      let!(:with_a_0_and_b_1) { create(:model, a: 0, b: 1) }
      let!(:with_a_1_and_b_0) { create(:model, a: 1, b: 0) }
      let!(:with_a_1_and_b_1) { create(:model, a: 1, b: 1) }

      it { should_not include(with_a_0_and_b_0) }
      it { should     include(with_a_0_and_b_1) }
      it { should     include(with_a_1_and_b_0) }
      it { should     include(with_a_1_and_b_1) }
    end
  end
end

But then it feels like I'm retesting .a and .b on the .a_or_b test, and if I compose it again, with yet another scope, it'll get bigger and bigger.

What is the sane way of dealing with this?

Also: is this a unit or integration test?