has_one through setter

model.rb

class User < ActiveRecord::Base
  has_many :memberships, inverse_of: :user
end

# ha anche il flag owner
class Membership < ActiveRecord::Base
  belongs_to :user,  inverse_of: :memberships
  belongs_to :group, inverse_of: :memberships
end

class Group < ActiveRecord::Base
  has_many :memberships, inverse_of: :group
  has_one  :owner_membership, -> { where owner: true }, class_name: 'Membership'
  has_one  :owner, source: :user, through: :owner_membership
end

model_spec.rb

require 'spec_helper'

describe Group do
  describe '#owner' do
    subject { Group.create! }

    context 'when there are no memberships' do
      it 'returns nil' do
        expect(subject.owner).to be_nil
      end
    end

    context 'when there are memberships' do
      let!(:owner) { Membership.create!(user: User.create!, group: subject, owner: true) }
      before { Membership.create!(user: User.create!, group: subject) }

      it 'returns a user' do
        expect(subject.owner).to be_a User
      end

      it 'returns the owner user' do
        expect(subject.owner).to eq owner.user
      end
    end
  end

  describe '#owner=' do
    subject { Group.create! }

    context 'when there are no memberships' do
      it 'adds a new membership' do
        expect {
          subject.owner = User.create!
        }.to change(Membership, :count).to 1
      end

      it 'sets the new owner' do
        user = User.create!
        subject.owner = user
        subject.reload # per sicurezza
        expect(subject.owner).to eq user
      end
    end
  end
end