Rails custom validation to ensure a Store always has an Owner - ruby-on-rails

Question

Logically, I'm trying to demote the a Member role from Owner to User. Expected behaviour is to produce an error "Store must have at least one owner".
It's use-case is to support the promotion of other members roles to Owner permissions, but never leave the store without at least one owner. I thought to add a validation to the Member for that, but it appears that isn't sufficient as it let's an Owner get demoted to a User (leaving the store without an Owner). Moreover, a new Owner cannot be set after since store_has_owner will return false.
class Admin::MembersController < Admin::BaseController
def make_user
member = current_company.members.find(params[:id])
if member.update(role: "user")
redirect_to admin_dashboard_path, notice: "#{member.user.email} is now a user."
else
redirect_to admin_dashboard_path, alert: member.errors
end
end
end
Member Exists? (0.5ms) SELECT 1 AS one FROM "members" WHERE "members"."store_id" = $1 AND "members"."role" = $2 LIMIT $3 [["store_id", 1], ["role", 0], ["LIMIT", 1]]
15:15:12 web.1 | ↳ app/models/member.rb:21:in `store_has_owner'
15:15:12 web.1 | Member Update (0.4ms) UPDATE "members" SET "role" = $1, "updated_at" = $2 WHERE "members"."id" = $3 [["role", 1], ["updated_at", "2019-08-26 19:15:12.965431"], ["id", 1]]
15:15:12 web.1 | ↳ app/controllers/admin/members_controller.rb:39:in `make_user'
15:15:12 web.1 | (0.6ms) COMMIT
15:15:12 web.1 | ↳ app/controllers/admin/members_controller.rb:39:in `make_user'
15:15:12 web.1 | Redirected to http://lvh.me:5000/admin/dashboard
15:15:12 web.1 | Completed 302 Found in 24ms (ActiveRecord: 3.8ms | Allocations: 9037)
Store
class Store < ApplicationRecord
has_many :members
has_many :users, through: :members
end
Member
class Member < ApplicationRecord
belongs_to :store
belongs_to :user
enum role: [ :owner, :user ]
validate :store_has_owner
def owner?
self.role == "owner"
end
def store_has_owner
errors.add(:member, "Store must have at least one owner.") unless store.members.owner.exists?
end
end
User
class User < ApplicationRecord
# Automatically remove the associated `members` join records
has_many :members, dependent: :destroy
has_many :stores, through: :members
end

Answer 1

unless clause should be
store.members.any? { |m| (m == self ? self : m).owner? }
There we're finding any member (m) in the members collection which owner? method returns true, i.e. Store has at least one Owner.
There is one caveat though. Accessing store.members causes selecting this relation from database and, because changed role of self is uncommitted yet, it returns that self is an Owner and this check don't work as expected. To avoid this we substitute m with self when they are equal, so m.owner? became self.owner? and returns false as expected.

Answer 2

Here are a few solutions I can think of:
1) This validation does not belongs to the member, but to the store. The store cannot be valid without an owner. Move it to the store and then add validates_associated :store to user.rb.
#store.rb
validate :has_owner
def has_owner
errors.add(:base, "Store must have at least one owner.") unless members.owner.any?
end
This should take care of the problem and is cleaner. Another solution follows. A bit dirtier but...
2) You can deal with this in the workflow of reducing a Member from Owner to User, by making sure there is another owner other than the member you are demoting. So:
# admin/members_controller.rb
def make_user
member = current_company.members.find(params[:id])
if member.update(role: "user") && current_company.has_other_owner_than(member)?
redirect_to admin_dashboard_path, notice: "#{member.user.email} is now a user."
else
redirect_to admin_dashboard_path, alert: member.errors
end
end
# store.rb
def has_other_owner_than?(member)
members.where(role: :owner).not(id: member.id).any? # Rails 5
# or
# members.where("role = ? AND id != ?", Member.roles[:owner], member.id).any? # Rails 4.2
end