class Headquarter < ApplicationRecord
  has_many :company_branches
end

class Branch < ApplicationRecord
  belongs_to :company_headquarter
end

RSpec.describe Headquarter, type: :model do
  it { should have_many(branches)}
end

RSpec.describe Branch, type: :model do
  it { should belong_to(:headquarter)}
end

RSpec.describe Company::Headquarter, type: :model do
  it { should have_many(:company_branches)}
end

Failure/Error: it { should have_many(:company_branches)}
   Expected Company::Headquarter to have a has_many association called company_branches (Company::Branch does not have a headquarter_id foreign key.)