blocks|key|790378|text|据我所知，这里没有rails的魔力，你必须自己来做。你可以使用一个虚拟属性来清理它...|type|unstyled|depth|inlineStyleRanges|entityRanges|data|790379|在模型类中：|790380|def+before_save
++@was_a_new_record+=+new_record?
++return+true
end

def+after_save
++if+@was_a_new_record
++++...
++end
end|code-block|syntax|javascript|790381|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

No rails magic here that I know of, you'll have to do it yourself. You could clean this up using a virtual attribute...

In your model class:

<pre><code>def before_save
 @was_a_new_record = new_record?
 return true
end

def after_save
 if @was_a_new_record
 ...
 end
end
</code></pre>

blocks|key|1004552|text|我想用它来做一个after_save回调。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1004553|一种更简单的解决方案是使用id_changed?+(因为它在update上不会改变)，甚至在存在时间戳列的情况下使用created_at_changed?。|1004554|更新:正如@mitsy指出的那样，如果需要在回调之外进行此检查，则使用id_previously_changed?。参见docs。|1004555|entityMap|0|LINK|mutability|MUTABLE|url|http://api.rubyonrails.org/classes/ActiveModel/Dirty.html#method-i-previous_changes^0|8|A|0|D|B|U|6|1M|J|0|Z|M|1O|4|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@$9|S|A|T|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|11|8|@$9|12|A|13|B|C]]|D|@$9|14|A|15|1|16]]|E|$]]|$1|J|3|-4|5|6|7|17|8|@]|D|@]|E|$]]]|K|$L|$5|M|N|O|E|$P|Q]]]]

I was looking to use this for an <code>after_save</code> callback.

A simpler solution is to use <code>id_changed?</code> (since it won't change on <code>update</code>) or even <code>created_at_changed?</code> if timestamp columns are present.

Update: As @mitsy points out, if this check is needed outside of callbacks then use <code>id_previously_changed?</code>. See <a href="http://api.rubyonrails.org/classes/ActiveModel/Dirty.html#method-i-previous_changes" rel="noreferrer">docs</a>.

blocks|key|1004620|text|由于该对象已保存，因此您需要查看之前的更改。ID应仅在创建后更改。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1004621|#+true+if+this+is+a+new+record
@object.previous_changes[:id].any?|code-block|syntax|javascript|1004622|还有一个实例变量@new_record_before_save。您可以通过执行以下操作来访问它：|offset|length|style|CODE|1004623|#+true+if+this+is+a+new+record
@object.instance_variable_get(:@new_record_before_save)|1004624|这两个都很难看，但是它们可以让你知道对象是否是新创建的。希望这能有所帮助！|1004625|entityMap^0|0|0|8|N|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@$I|V|J|W|K|L]]|9|@]|A|$]]|$1|M|3|N|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|Y|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

Since the object has already been saved, you would you need to look at the previous changes. The ID should only change after a create.

<pre><code># true if this is a new record
@object.previous_changes[:id].any?
</code></pre>

There is also an instance variable <code>@new_record_before_save</code>. You can access that by doing the following:

<pre><code># true if this is a new record
@object.instance_variable_get(:@new_record_before_save)
</code></pre>

Both are pretty ugly, but they would allow you to know whether the object has been newly created. Hope that helps!

blocks|key|1004680|text|Rails+5.1%2B方式：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1004681|user+=+User.new
user.save!
user.saved_change_to_attribute?(:id)+#+=>+true|code-block|syntax|javascript|1004682|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Rails 5.1+ way:

<pre><code>user = User.new
user.save!
user.saved_change_to_attribute?(:id) # =&gt; true
</code></pre>

blocks|key|1004721|text|对于Rails4(在4.2.11.1上检查)，在after_save中创建对象时，changes和previous_changes方法的结果是空的散列{}。因此，像id_changed?这样的attribute_changed?方法不会像预期的那样工作。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1004722|但是您可以利用这些知识-知道在更新时changes中至少必须有一个属性-检查changes是否为空。一旦你确认它是空的，你必须在对象创建过程中：|1004723|after_save+do
++if+changes.empty?
++++#+code+appropriate+for+object+creation+goes+here+...
++end
end|code-block|syntax|javascript|1004724|entityMap^0|O|A|15|7|1D|G|23|2|2A|B|2O|I|0|I|7|12|7|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|11|8|@$9|12|A|13|B|C]|$9|14|A|15|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|16|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|17|8|@]|D|@]|E|$]]]|N|$]]

For Rails 4 (checked on 4.2.11.1) results of <code>changes</code> and <code>previous_changes</code> methods are empty hashes <code>{}</code> on object creation inside <code>after_save</code>. So <code>attribute_changed?</code> methods like <code>id_changed?</code> won't work as expected.

But you can take advantage of this knowledge and - knowing that at least 1 attribute has to be in <code>changes</code> on update - check if <code>changes</code> is empty. Once you confirm that it's empty, you must be during object creation:

<pre><code>after_save do
 if changes.empty?
 # code appropriate for object creation goes here ...
 end
end
</code></pre>

blocks|key|790762|text|对于这种用例，有一个名为previously_new_record?的方法。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|790763|user+=+User.new

user.new_record?+#+=>+true
user.previously_new_record?+#+=>+false

user.save

user.new_record?+#+=>+false
user.previously_new_record?+#+=>+true|code-block|syntax|javascript|790764|来源：https://api.rubyonrails.org/v6.1.4/classes/ActiveRecord/Persistence.html#method-i-previously_new_record-3F|790765|看起来建议的通过调用saved_change_to_id?的解决方法不再起作用。我使用的是Rails+7。|790766|entityMap|0|LINK|mutability|MUTABLE|url|https://api.rubyonrails.org/v6.1.4/classes/ActiveRecord/Persistence.html#method-i-previously_new_record-3F^0|C|M|0|0|3|2Y|0|0|A|J|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|Z|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|10|8|@]|D|@$9|11|A|12|1|13]]|E|$]]|$1|M|3|N|5|6|7|14|8|@$9|15|A|16|B|C]]|D|@]|E|$]]|$1|O|3|-4|5|6|7|17|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]]]

There is a method called <code>previously_new_record?</code> for exactly this use case.
<pre class="lang-rb prettyprint-override"><code>user = User.new

user.new_record? # =&gt; true
user.previously_new_record? # =&gt; false

user.save

user.new_record? # =&gt; false
user.previously_new_record? # =&gt; true
</code></pre>
Source: <a href="https://api.rubyonrails.org/v6.1.4/classes/ActiveRecord/Persistence.html#method-i-previously_new_record-3F" rel="nofollow noreferrer">https://api.rubyonrails.org/v6.1.4/classes/ActiveRecord/Persistence.html#method-i-previously_new_record-3F</a>
Looks like the proposed workaround by calling <code>saved_change_to_id?</code> doesn't work anymore. I'm on Rails 7.

The #new_record? function determines if a record has been saved. But it is always false in the <code>after_save</code> hook. Is there a way to determine whether the record is a newly created record or an old one from update? 

I'm hoping not to use another callback such as <code>before_create</code> to set a flag in the model or require another query into the db.

Any advice is appreciated. 

Edit: Need to determine it in <code>after_save</code> hook, and for my particular use case, there is no <code>updated_at</code> or <code>updated_on</code> timestamp

How to determine if a record is just created or updated in after_save

数据库

#new_record？函数确定记录是否已保存。但在after_save钩子中，它始终是假的。有没有一种方法可以确定记录是新创建的记录还是更新中的旧记录？我希望不要使用另一个回调函数，比如before_create来在模型中设置一个标志，或者需要对数据库进行另一个查询。任何建议都是值得感谢的。编辑:需要在after_s...

问如何确定记录是刚在after_save中创建还是更新
EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问如何确定记录是刚在after_save中创建还是更新EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问如何确定记录是刚在after_save中创建还是更新
EN