model_refactoring.rb

# before (legacy model)
# app/models/user.rb
class User < ActiveRecord::Base
  validates :name, length: { maximum: 100 }
  validates :lastname, length: { maximum: 100 }
  validates :patronymic, length: { maximum: 100 }
  
  before_save :capitalize_name!, :if => :name_process_needed?
  
  private

  def name_process_needed?
    name_changed? || lastname_changed? || patronymic_changed?
  end
  
  def capitalize_name!
    self.name = capitalize_string(self.name) if( name_changed? && !self.name.blank? )
    self.lastname = capitalize_string(self.lastname) if( lastname_changed? && !self.lastname.blank? )
    self.patronymic = capitalize_string(self.patronymic) if( patronymic_changed? && !self.patronymic.blank?)
  end

  def capitalize_string str
    new_str = str.slice(0,1).mb_chars.capitalize + str.slice(1..-1).mb_chars.downcase
    new_str.to_s
  end
end

# after
# app/models/user.rb
class User < ActiveRecord::Base
  validates :name, length: { maximum: 100 }
  validates :lastname, length: { maximum: 100 }
  validates :patronymic, length: { maximum: 100 }
end

# app/mutators/user_mutator.rb
class UserMutator
  def self.create(params)
    params = params
                 .merge(agreement: true)
                 .merge(capitalize_fio(params))

    item = User.new(params)
    item.save
    item
  end

  def self.update(user_id, params)
    user = User.find(user_id)
    params = params.merge(capitalize_fio(params))

    user.update(params)
    user
  end

  # params['name'] || params[:name] - it looks like unsolved problem outside
  def self.capitalize_fio(params)
    {
      name: StringOperations.my_capitalize(params[:name]),
      lastname: StringOperations.my_capitalize(params[:lastname]),
      patronymic: StringOperations.my_capitalize(params[:patronymic]),
    }
  end
end

#lib/string_oprations.rb
module StringOperations
  def my_capitalize(str)
    return '' if str.nil? or str.size.zero?
    str.slice(0, 1).mb_chars.capitalize + str.slice(1..-1).mb_chars.downcase
  end
end

# app/controllers/admin/api/users_controller.rb
module Admin
  module Api
    class UsersController < AdminController
      def create
        @item = UserMutator.create(item_params)
        if @item.errors
          return render_json_errors @item.errors
        else
          render :show
        end
      end

      def update
        @item = UserMutator.update(params[:id], item_params)
        if @item.errors.empty?
          render_json_errors @item.errors
        else
          render :show
        end
      end

      private

      def item_params
        params.require(:users).permit(
          :role_id, :name, :lastname, :patronymic, :email, :password,
          :password_confirmation
        )
      end
    end
  end
end