Rails: :save 返回 NoMethodError答案

【问题标题】：Rails: :save returns NoMethodErrorRails: :save 返回 NoMethodError
【发布时间】：2019-05-20 05:36:39
【问题描述】：

我正在编写一个用于练习的小型 Rails api。目标是发布一些数据，然后将其（经过进一步处理）保存在数据库中。对象正在成功创建。在保存方法上出现我无法解释的错误：

NoMethodError (undefined method `[]' for nil:NilClass): app/controllers/api/v1/calculations_controller.rb:19:in `create'

它出现在 :create 方法中（在 POST 上）：

  # POST /calculations
  def create
    @calculation = Calculation.new(calculation_params)
    if @calculation.save # <- This is line 19
      render json: @calculation, status: :created, location: @calculation
    else
      render json: @calculation.errors, status: :unprocessable_entity
    end
  end

模型如下所示：

require 'digest'

class Calculation < ApplicationRecord

  attr_reader :value, :hash_value, :algorithm, :timestamp
  @@known_algorithms = ['SHA2_256', 'SHA2_384', 'SHA2_512']

  def initialize (params)
    @value = params[:value]
    @algorithm = params[:algorithm]
    validate!
    @hash_value, @timestamp = digest
  end

  private

  def digest
    case @algorithm
    when 'SHA2_256'
      result = Digest::SHA2.hexdigest(@value)
    when 'SHA2_384'
      result = Digest::SHA2.new(384).hexdigest(@value)
    when 'SHA2_512'
      result = Digest::SHA2.new(512).hexdigest(@value)
    end
    return result, DateTime.now
  end

  def validate!
    raise ArgumentError.new('Value cannot be empty or NIL!') if @value.nil? or @value.empty?
    raise ArgumentError.new('Unknown hashing algorithm!') unless @@known_algorithms.include? @algorithm
  end
end

为什么会出现NoMethodError？我是否覆盖了模型类中的某些重要部分？

根据要求进行完整的堆栈跟踪：

/home/pbz/.gem/ruby/2.5.0/gems/activerecord-5.2.2/lib/active_record/transactions.rb:424:in `clear_transaction_record_state'
/home/pbz/.gem/ruby/2.5.0/gems/activerecord-5.2.2/lib/active_record/transactions.rb:330:in `ensure in rollback_active_record_state!'
/home/pbz/.gem/ruby/2.5.0/gems/activerecord-5.2.2/lib/active_record/transactions.rb:330:in `rollback_active_record_state!'
/home/pbz/.gem/ruby/2.5.0/gems/activerecord-5.2.2/lib/active_record/transactions.rb:309:in `save'
/home/pbz/.gem/ruby/2.5.0/gems/activerecord-5.2.2/lib/active_record/suppressor.rb:44:in `save'
/home/pbz/RubymineProjects/hasher_api/app/controllers/api/v1/calculations_controller.rb:20:in `create'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_controller/metal/basic_implicit_render.rb:6:in `send_action'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/abstract_controller/base.rb:194:in `process_action'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_controller/metal/rendering.rb:30:in `process_action'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/abstract_controller/callbacks.rb:42:in `block in process_action'
/home/pbz/.gem/ruby/2.5.0/gems/activesupport-5.2.2/lib/active_support/callbacks.rb:132:in `run_callbacks'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/abstract_controller/callbacks.rb:41:in `process_action'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_controller/metal/rescue.rb:22:in `process_action'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_controller/metal/instrumentation.rb:34:in `block in process_action'
/home/pbz/.gem/ruby/2.5.0/gems/activesupport-5.2.2/lib/active_support/notifications.rb:168:in `block in instrument'
/home/pbz/.gem/ruby/2.5.0/gems/activesupport-5.2.2/lib/active_support/notifications/instrumenter.rb:23:in `instrument'
/home/pbz/.gem/ruby/2.5.0/gems/activesupport-5.2.2/lib/active_support/notifications.rb:168:in `instrument'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_controller/metal/instrumentation.rb:32:in `process_action'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_controller/metal/params_wrapper.rb:256:in `process_action'
/home/pbz/.gem/ruby/2.5.0/gems/activerecord-5.2.2/lib/active_record/railties/controller_runtime.rb:24:in `process_action'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/abstract_controller/base.rb:134:in `process'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_controller/metal.rb:191:in `dispatch'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_controller/metal.rb:252:in `dispatch'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_dispatch/routing/route_set.rb:52:in `dispatch'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_dispatch/routing/route_set.rb:34:in `serve'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_dispatch/journey/router.rb:52:in `block in serve'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_dispatch/journey/router.rb:35:in `each'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_dispatch/journey/router.rb:35:in `serve'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_dispatch/routing/route_set.rb:840:in `call'
/home/pbz/.gem/ruby/2.5.0/gems/rack-2.0.6/lib/rack/etag.rb:25:in `call'
/home/pbz/.gem/ruby/2.5.0/gems/rack-2.0.6/lib/rack/conditional_get.rb:38:in `call'
/home/pbz/.gem/ruby/2.5.0/gems/rack-2.0.6/lib/rack/head.rb:12:in `call'
/home/pbz/.gem/ruby/2.5.0/gems/activerecord-5.2.2/lib/active_record/migration.rb:559:in `call'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_dispatch/middleware/callbacks.rb:28:in `block in call'
/home/pbz/.gem/ruby/2.5.0/gems/activesupport-5.2.2/lib/active_support/callbacks.rb:98:in `run_callbacks'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_dispatch/middleware/callbacks.rb:26:in `call'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_dispatch/middleware/executor.rb:14:in `call'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_dispatch/middleware/debug_exceptions.rb:61:in `call'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_dispatch/middleware/show_exceptions.rb:33:in `call'
/home/pbz/.gem/ruby/2.5.0/gems/railties-5.2.2/lib/rails/rack/logger.rb:38:in `call_app'
/home/pbz/.gem/ruby/2.5.0/gems/railties-5.2.2/lib/rails/rack/logger.rb:26:in `block in call'
/home/pbz/.gem/ruby/2.5.0/gems/activesupport-5.2.2/lib/active_support/tagged_logging.rb:71:in `block in tagged'
/home/pbz/.gem/ruby/2.5.0/gems/activesupport-5.2.2/lib/active_support/tagged_logging.rb:28:in `tagged'
/home/pbz/.gem/ruby/2.5.0/gems/activesupport-5.2.2/lib/active_support/tagged_logging.rb:71:in `tagged'
/home/pbz/.gem/ruby/2.5.0/gems/railties-5.2.2/lib/rails/rack/logger.rb:26:in `call'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_dispatch/middleware/remote_ip.rb:81:in `call'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_dispatch/middleware/request_id.rb:27:in `call'
/home/pbz/.gem/ruby/2.5.0/gems/rack-2.0.6/lib/rack/runtime.rb:22:in `call'
/home/pbz/.gem/ruby/2.5.0/gems/activesupport-5.2.2/lib/active_support/cache/strategy/local_cache_middleware.rb:29:in `call'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_dispatch/middleware/executor.rb:14:in `call'
/home/pbz/.gem/ruby/2.5.0/gems/actionpack-5.2.2/lib/action_dispatch/middleware/static.rb:127:in `call'
/home/pbz/.gem/ruby/2.5.0/gems/rack-2.0.6/lib/rack/sendfile.rb:111:in `call'
/home/pbz/.gem/ruby/2.5.0/gems/railties-5.2.2/lib/rails/engine.rb:524:in `call'
/home/pbz/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/puma-3.12.0/lib/puma/configuration.rb:225:in `call'
/home/pbz/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/puma-3.12.0/lib/puma/server.rb:658:in `handle_request'
/home/pbz/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/puma-3.12.0/lib/puma/server.rb:472:in `process_client'
/home/pbz/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/puma-3.12.0/lib/puma/server.rb:332:in `block in run'
/home/pbz/.rbenv/versions/2.5.3/lib/ruby/gems/2.5.0/gems/puma-3.12.0/lib/puma/thread_pool.rb:133:in `block in spawn_thread'

【问题讨论】：

您能否检查一下您是否在calculation_params 中获得了有效数据？
你能发布完整的堆栈跟踪吗？
@mehedi 我得到了有效的参数。访问和打印计算对象的成员时，我得到了正确的值。
@engineersmnky 更新了帖子

标签： ruby-on-rails json ruby post

【解决方案1】：

不要覆盖AR 中的initialize。它可能会破坏模型中的很多东西。

您可以使用after_initialize。

每当有 Active Record 时，都会调用 after_initialize 回调对象被实例化，通过直接使用 new 或当记录是从数据库中加载的。 避免需要直接覆盖你的 Active Record initialize 方法。

def after_initialize
  # Gets called right after Calculation.new
  # Do some stuff here
end

如果您仍想使用initialize，您必须执行以下操作：

def initialize(attributes = {})
  super
  self.attributes = attributes
end

【讨论】：

感谢您的建议，但我发现了有关 ActiveRecords work 以及如何访问它的字段的更多信息。与 after_initialize 一起一切正常。我现在将发布我自己的答案并接受它。还是谢谢你！
@PhilippB upvote 是你能说的最好的谢谢。
我投了赞成票，但由于我的声望很低，所以没有显示。

【解决方案2】：

我确实发现（感谢@Зелёный），在创建模型后为模型创建值（new）我需要使用 Rails 提供的after_initialize 回调。与this comment 一起了解如何访问模型实例变量，我能够实现我想要的功能。最初的 NoMethodError 是我错误使用 initialize 方法的结果。以下代码是最终结果，适用于默认控制器。

require 'digest'

class Calculation < ApplicationRecord

  @@known_algorithms = ['SHA2_256', 'SHA2_384', 'SHA2_512']

  after_initialize :init

  private

  def init
    validate!
    calculate!
  end

  def calculate!
    self.hash_value, self.timestamp = digest
  end

  def digest
    case algorithm
    when 'SHA2_256'
      result = Digest::SHA2.hexdigest(value)
    when 'SHA2_384'
      result = Digest::SHA2.new(384).hexdigest(value)
    when 'SHA2_512'
      result = Digest::SHA2.new(512).hexdigest(value)
    end
    return result, DateTime.now
  end

  def validate!
    raise ArgumentError.new('Value cannot be empty or NIL!') if value.nil? or value.empty?
    raise ArgumentError.new('Unknown hashing algorithm!') unless @@known_algorithms.include? algorithm
  end
end

【讨论】：