instituions_controller.rb

  def create
    @institution = Institution.new(institution_params.except(:admin), as: :admin)
    if @institution.save_with_admin(login: institution_params[:admin] 
      redirect_to admin_institutions_path, flash: {
        success: "Institution created."
      }
    else
      flash.now[:error] = 'Unable to create institution.'
      render :new
    end
  end

institution.rb

  def save_with_admin(login: login)
    admin = User.find_by_login(login)
    if admin.present? || login.nil?
      self.admin = admin
      save
    else
      errors.add(:admin_id, "Could not find user with login #{login}")
      false
    end
  end