application_controller.rb

# app/controllers/application_controller.rb

class ApplicationController < ActionController::Base
  before_action :authenticate_user!
  include CurrentTenant
end

current.rb

# app/models/current.rb

class Current < ActiveSupport::CurrentAttributes
  attribute :tenant, :user, :membership

  resets { Time.zone = nil }

  def membership=(membership)
    super
    self.user   ||= membership.try(:user)
    self.tenant ||= membership.try(:tenant)
    # Time.zone = user.time_zone
  end
end

current_tenant.rb

# app/controllers/concerns/current_tenant.rb

module CurrentTenant
  extend ActiveSupport::Concern

  included do
    before_action :current_membership, if: :user_signed_in?
    before_action :require_tenant!,    if: :user_signed_in?

    private

    # rubocop:disable Metrics/AbcSize
    def current_membership
      Current.membership ||= if cookies[:tenant_id]
                               current_user.memberships.where(tenant_id: cookies[:tenant_id]).take
                             else
                               current_user.memberships.count == 1 ? current_user.memberships.first : nil
                             end
    end
    # rubocop:enable Metrics/AbcSize

    def require_tenant!
      redirect_to(tenants_path, notice: t('Please select a tenant')) && return unless current_tenant
    end

    def current_tenant
      Current.tenant ||= Tenant.find_by id: cookies[:tenant_id]
    end

    helper_method :current_tenant, :current_membership
  end
end

routes.rb

Rails.application.routes.draw do
  # List tenants...
  get '/tenants', to: 'tenants#index', as: 'tenants'
  # Switch tenants...
  get '/tenants/:id', to: 'tenants#show', as: 'switch_tenant'
end

tenant_controller.rb

class TenantsController < ApplicationController
  skip_before_action :require_tenant!, only: %i[index show]
  
  # Show a list of the tenants that I have access to: 
  def index
    @memberships = current_user.memberships
  end

  # Switch tenants: 
  # Set a cookie, then redirect to that tenant.
  def show
    tenant = current_user.tenants.find(params[:id])
    cookies.permanent[:tenant_id] = tenant.id
    redirect_to(session[:return_to] || params[:return_to] || root_path)
  end
end