`lazy_find_by` returns AR instance that executes queries when attributes is needed.

_how_to_use.rb

## Simple UseCase

user = User.lazy_find_by(id: 1, name: "foo")
#=> #<User id: 1, name: "foo", age: nil, created_at: nil, updated_at: nil>
user.id
#=> 1
user.name
#=> "foo"
user.age
  User Load (0.4ms)  SELECT "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
#=> 1

## Use has_many

user_2 = User.lazy_find_by(id: 1)
user_2.posts.find_by(id: 1)
  Post Load (0.3ms)  SELECT "posts".* FROM "posts" WHERE "posts"."user_id" = ? AND "posts"."id" = ? LIMIT ?  [["user_id", 1], ["id", 1], ["LIMIT", 1]]
#=> #<Post id: 1, user_id: 1, title: "bar", created_at: "2019-05-10 15:44:39", updated_at: "2019-05-10 15:44:39">

user_3 = User.lazy_find_by(id: 1)
post = user_3.posts.create(title: "buz")
  User Load (3.5ms)  SELECT "users".* FROM "users" WHERE "users"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
   (0.1ms)  begin transaction
  Post Create (3.0ms)  INSERT INTO "posts" ("user_id", "title", "created_at", "updated_at") VALUES (?, ?, ?, ?)  [["user_id", 1], ["title", "buz"], ["created_at", "2019-05-10 17:12:06.418763"], ["updated_at", "2019-05-10 17:12:06.418763"]]
   (1.5ms)  commit transaction
#=> #<Post id: 4, user_id: 1, title: "buz", created_at: "2019-05-10 17:12:06", updated_at: "2019-05-10 17:12:06">

active_record_lazy_find_by.rb

# frozen_string_literal: true

module ActiveRecordLazyFindBy
  STATE_METHOD_NAMES = %i[valid? new_record? persisted?].freeze

  class << self
    def module_for(klass, attributes)
      lazy_attr_names = (klass.attribute_names - attributes.keys.map(&:to_s)).sort
      key = [klass, lazy_attr_names]

      cache[key] ||= build(lazy_attr_names)
    end

    def cache
      @cache ||= {}
    end

    private

    def build(attr_names)
      lazy_method_names = attr_names.flat_map { |x| [x, "#{x}?", "#{x}="] } + STATE_METHOD_NAMES

      Module.new do
        lazy_method_names.each do |m|
          define_method(m) do |*args|
            unless @lazy_new_record
              @lazy_new_record = true
              reload
            end
            super(*args)
          end
        end
      end
    end
  end

  module Methods
    def self.included(base)
      base.extend ClassMethods
    end

    module ClassMethods
      def lazy_find_by(attributes = {})
        mod = ActiveRecordLazyFindBy.module_for(self, attributes)
        new(attributes).extend(mod)
      end
    end
  end
end

application_record.rb

class ApplicationRecord < ActiveRecord::Base
  include ActiveRecordLazyFindBy::Methods

  self.abstract_class = true
end