a_foreign_key_constraint_association.rb

RSpec.shared_examples('a foreign_key constraint association') do
  specify do
    expect do
      build(described_model, "#{association}_id" => 0).save(validate: false)
    end.to raise_error(ActiveRecord::InvalidForeignKey)
  end
end

employee_spec.rb

RSpec.describe(Employee) do
  describe('associations') do
    it_behaves_like('a foreign_key constraint association') do
      let(:association) { :employer }
    end
  end
end