blocks|key|1619423|text|通常，当人们认为他们需要这样做时，他们实际上并不需要这样做。就像John说的，解释你正在尝试做什么，别人可能会建议一种方法，而不需要事先知道id。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1619424|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Usually when people think they need to do this they actually do not need to do it. Like John says, explain what you are trying to do and someone can probably suggest a way to do it without having to know the id in advance.

blocks|key|876466|text|我也在找这个，我找到了答案：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|876467|假设模型名为"+model“，表名为"models”|876468|model.rb|876469|before_save+{
++++next_id=Model.connection.select_value("Select+nextval('models_id_seq')")
}|code-block|syntax|javascript|876470|这将输出您的记录在保存时将作为id的值。|876471|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|Q|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|R|8|@]|9|@]|A|$I|J]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

I was looking for this too, and I found an answer:

Let's suppose model name is "Model" and table name is "models"

model.rb

<pre><code>before_save {
 next_id=Model.connection.select_value("Select nextval('models_id_seq')")
}
</code></pre>

This will output the value your record will take for id IF it gets saved

blocks|key|876533|text|这与其说是一个Rails问题，不如说是一个数据库问题。这是一个在任何web应用程序框架中都会出现的问题，并且解决方案在所有地方都是相同的。您必须使用数据库事务。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|876534|基本流程是这样工作的。|876535|876536|打开transaction|unordered-list-item|876537|Save您的模型|876538|使用数据库分配的ID+|876539|如果您实际上不想将此模型保留在数据库中，请回滚transaction.|876540|If如果您想要将模型保留在数据库中，请提交transaction.|876541|876542|从这种方法中，您将注意到的主要内容是，在回滚事务的in中将存在间隙。|876543|entityMap^0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|W|8|@]|9|@]|A|$]]|$1|E|3|F|5|G|7|X|8|@]|9|@]|A|$]]|$1|H|3|I|5|G|7|Y|8|@]|9|@]|A|$]]|$1|J|3|K|5|G|7|Z|8|@]|9|@]|A|$]]|$1|L|3|M|5|G|7|10|8|@]|9|@]|A|$]]|$1|N|3|O|5|G|7|11|8|@]|9|@]|A|$]]|$1|P|3|-4|5|6|7|12|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|13|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|14|8|@]|9|@]|A|$]]]|T|$]]

This is less a Rails question and more a database question. This is a problem that will present itself in any web application framework, and the solution is the same in all places. You have to use a database transaction.

The basic flow will work like this.

<ul>
<li>Open a transaction</li>
<li>Save your model</li>
<li>Use the ID assigned by the database</li>
<li>If it turns out you actually don't want to keep this model in the database, roll back the transaction.</li>
<li>If it turns out you want to keep the model in the database, commit the transaction.</li>
</ul>

The main thing you will notice from this approach is that there will be gaps in your IDs where you rolled back the transaction.

blocks|key|874240|text|使用自动递增整数主键的默认Rails约定，无法在保存之前获取模型的ID，因为它是由RDBMS在相关表中插入新行时生成的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|874241|你真正想解决的问题是什么？|874242|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

Using the default Rails convention of an auto-incrementing integer primary key, there's no way to get the ID of a model before it's saved because it's generated by the RDBMS when the new row is inserted in the relevant table.

What problem are you actually trying to solve?

blocks|key|876559|text|大多数情况下，当我需要一个id时，可以将其分组到一个简短的列表中。通过创建嵌套关联或关联的连接时。假设我们有:通过:user_pets关联具有:pets的:user，我们将在其中保存它们的类型。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|876560|如果我们有一个正确配置的"has_many:+through+Association“，我们可以只使用User.pets.create(name:+"Rex")，但这太简单了，因为我们想要在:user_pets中创建:pet类型。|876561|u+=+User.create(name:+"Cesar")
u.id+#+=>+1+#+works+fine

p+=+u.pets.create(name:+'Rex')+
#+=>+rails+will+create+UserPets+=>+{id:+1,+user_id:+1,+pet_id:+1}+for+us

#+But+now+we+have+a+problem,+how+do+we+find+:user_pets+of+our+:pet?
#+remember+we+still+need+to+update+the+:type,+the+ugly+(wrong)+way:
up+=+p.user_pets.first
up.type+=+'dog'
up.save!+#+working+but+wrong

#+Do+you+see+the+problems+here?+We+could+use+an+id
P+=+Pet.new(+name:+"Destroyer"+)
p.id+#+will+not+work,+as+the+pet+not+saved+yet+to+receive+an+id
up+=+UserPet.new(+user_id:+U.id,+pet_id:+p.id+)+
#+=>+UserPet+{id:+2,+user_id:+1,+pet_id:+nil}+#+as+we+expected+there+is+no+id.

#+What+solutions+do+we+have?+Use+nested+creation!
#+Good
up+=+UserPet.new(user_id:+u.id,+type:+"dog")
#+even+better
up+=+u.user_pets.new(type:+"dog")+
#+it's+just+a+shortcut+for+the+code+above,+
#+it+will+add+:user_id+for+us,+so+let's+just+remember+it.

#+Now+lets+add+another+'new'+from+our+creatd+'user_pet'
p+=+up.pets.new(name:+"Millan")
user.save!
#+=>+UserPet:+{id:+3,+user_id:+1,+pet_id:+2,+type:+'dog'}+#+=>+Pet:+{id:+2,+name:+"Sam"}
#+everything+is+working!+YEY!

#+we+can+even+better,+than+writing+in+the+beginning+"User.create",+
#+we+can+write+"User.new"+and+save+it+with+all+the+nested+elements.|code-block|syntax|javascript|876562|你看到这是如何为我们创建所有的ids了吗？让我们转到更复杂的东西！现在，我们有了一个与:user_pet完全相同的附加表:shampoo，它属于一个:pet和一个:user。我们需要在不知道:user和:pet的id的情况下创建它|876563|u+=+User.new('Mike')
up+=+u.user_pets.new(type:+"cat")+
p+=+up.pets.new(name:+"Rowe")

#+But+what+are+we+doing+now?
#+If+we+do:
s+=+u.shampoos.new(name:+"Dirty+Job")+
#+=>+Shampoo:+{user_id:+2,+pet_id:+nil,+name:+"..."}
#+We+get+"pet_id:+nil",+not+what+we+want.

#+Or+if+we+do:
s+=+p.shampoos.new(name:+"Schneewittchen")+
#+=>+Shampoo:+{user_id:+nil,+pet_id:+3,+name:+"..."}
#+We+get+"user_id:+nil",+in+both+cases+we+do+not+get+what+we+want.

#+So+we+need+to+get+the+id+of+not+created+record,+again.
#+For+this+we+can+just+do+as+in+the+first+example+(order+is+not+important)
s+=+u.shampoos.new(name:+"Mission+Impossible")+
#+=>+Shampoo:+{id:+3,+user_id:+2,+pet_id:+nil,+name:+"..."}
s.pet+=+p+#+this+will+give+the+missing+id,+to+the+shampoo+on+save.
#+Finish+with+save+of+the+object:
u.save!+#=>+Shampoo:+{id:+3,+user_id:+2,+pet_id:+3,+name:+'...'}+#+=>+Pet:+...
#+Done!|876564|我试着介绍了当你需要元素id时最常见的原因，以及如何克服这个问题。我希望这将是有用的。|876565|entityMap^0|1L|A|1Z|5|25|5|0|1F|T|2N|A|30|4|0|0|17|9|1O|8|22|4|29|5|2N|5|2T|4|0|0|0^^$0|@$1|2|3|4|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|F|3|G|5|6|7|11|8|@$9|12|A|13|B|C]|$9|14|A|15|B|C]|$9|16|A|17|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|18|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|19|8|@$9|1A|A|1B|B|C]|$9|1C|A|1D|B|C]|$9|1E|A|1F|B|C]|$9|1G|A|1H|B|C]|$9|1I|A|1J|B|C]|$9|1K|A|1L|B|C]]|D|@]|E|$]]|$1|O|3|P|5|J|7|1M|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|1N|8|@]|D|@]|E|$]]|$1|S|3|-4|5|6|7|1O|8|@]|D|@]|E|$]]]|T|$]]

Most of the time when I needed an id can be grouped into a short list.
When creating nested associations or connectin of the associations through.
Let's assume we have: <code>:user</code> that have <code>:pets</code> through <code>:user_pets</code> association, where we will save their type.

If we have a properly configured "has_many: through Association" we can just
<code>User.pets.create(name: "Rex")</code> but this is too simplistic, as we want to creat <code>:pet</code> type in <code>:user_pets</code>.

<pre><code>u = User.create(name: "Cesar")
u.id # =&gt; 1 # works fine

p = u.pets.create(name: 'Rex') 
# =&gt; rails will create UserPets =&gt; {id: 1, user_id: 1, pet_id: 1} for us

# But now we have a problem, how do we find :user_pets of our :pet?
# remember we still need to update the :type, the ugly (wrong) way:
up = p.user_pets.first
up.type = 'dog'
up.save! # working but wrong

# Do you see the problems here? We could use an id
P = Pet.new( name: "Destroyer" )
p.id # will not work, as the pet not saved yet to receive an id
up = UserPet.new( user_id: U.id, pet_id: p.id ) 
# =&gt; UserPet {id: 2, user_id: 1, pet_id: nil} # as we expected there is no id.

# What solutions do we have? Use nested creation!
# Good
up = UserPet.new(user_id: u.id, type: "dog")
# even better
up = u.user_pets.new(type: "dog") 
# it's just a shortcut for the code above, 
# it will add :user_id for us, so let's just remember it.

# Now lets add another 'new' from our creatd 'user_pet'
p = up.pets.new(name: "Millan")
user.save!
# =&gt; UserPet: {id: 3, user_id: 1, pet_id: 2, type: 'dog'} # =&gt; Pet: {id: 2, name: "Sam"}
# everything is working! YEY!

# we can even better, than writing in the beginning "User.create", 
# we can write "User.new" and save it with all the nested elements.
</code></pre>

You saw how this created all the ids for us? Let's move to something even more complex!
Now we have an additional table <code>:shampoo</code> that exactly as <code>:user_pet</code>, belongs to a <code>:pet</code> and a <code>:user</code>
We need to create it without knowing the id of the <code>:user</code> and <code>:pet</code>

<pre><code>u = User.new('Mike')
up = u.user_pets.new(type: "cat") 
p = up.pets.new(name: "Rowe")

# But what are we doing now?
# If we do:
s = u.shampoos.new(name: "Dirty Job") 
# =&gt; Shampoo: {user_id: 2, pet_id: nil, name: "..."}
# We get "pet_id: nil", not what we want.

# Or if we do:
s = p.shampoos.new(name: "Schneewittchen") 
# =&gt; Shampoo: {user_id: nil, pet_id: 3, name: "..."}
# We get "user_id: nil", in both cases we do not get what we want.

# So we need to get the id of not created record, again.
# For this we can just do as in the first example (order is not important)
s = u.shampoos.new(name: "Mission Impossible") 
# =&gt; Shampoo: {id: 3, user_id: 2, pet_id: nil, name: "..."}
s.pet = p # this will give the missing id, to the shampoo on save.
# Finish with save of the object:
u.save! #=&gt; Shampoo: {id: 3, user_id: 2, pet_id: 3, name: '...'} # =&gt; Pet: ...
# Done!
</code></pre>

I tried to cover most common causes when you need element id, and how to overcome this. I hope it will be useful.

blocks|key|4123683|text|我不相信有任何解决办法，因为id实际上是由数据库本身生成的。在将对象保存到数据库之前，id应该是不可用的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4123684|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

I don't believe there are any workarounds since the id is actually generated by the database itself. The id should not be available until after the object has been saved to the database.

blocks|key|876518|text|请考虑在创建实例后立即执行您想要的操作。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|876519|after_create+do
++print+self.id
end|code-block|syntax|javascript|876520|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|$]]

Consider doing what you want right after the instance is created.

<pre><code>after_create do
 print self.id
end
</code></pre>

blocks|key|1619563|text|首先了解数据库的结构。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1619564|使用sequence|1619565|increment+done+by+1(在创建序列时指定)生成|unordered-list-item|1619566|Id数据库的最后一个条目将具有最高的id|offset|length|style|CODE|1619567|值|1619568|1619569|如果你想得到要保存的记录的id，|1619570|然后，您可以使用以下命令：|1619571|+1.+id+=+Model.last.id+%2B+1
++++model+=+Model.new(id:+id)
++++model.save
++++#+But+if+data+can+be+delete+from+that+dataabse+this+will+not+work+correctly.
++
+2.+id+=+Model.connection.select_value("Select+nextval('models_id_seq')")
++++model+=+Model.new(id:+id)
++++model.save
++++#+Here+in+this+case+if+you+did+not+specified+'id'+while+creating+new+object,+record+will+saved+with+next+id.+
++++
++++e.g.+
++++id
++++=>+2234
++++model+=+Model.new(id:+id)+
++++model.save+
++++#+Record+will+be+created+using+'id'+as+2234++
++++
++++model+=+Model.new()
++++model.save
++++#+Record+will+be+created+using+next+value+of+'id'+as+2235++|code-block|syntax|javascript|1619572|希望这能对你有所帮助。|1619573|entityMap^0|0|0|0|I|2|0|0|0|D|2|0|0|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|13|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|14|8|@]|9|@]|A|$]]|$1|G|3|H|5|F|7|15|8|@$I|16|J|17|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|18|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|19|8|@]|9|@]|A|$]]|$1|P|3|Q|5|6|7|1A|8|@$I|1B|J|1C|K|L]]|9|@]|A|$]]|$1|R|3|S|5|6|7|1D|8|@]|9|@]|A|$]]|$1|T|3|U|5|V|7|1E|8|@]|9|@]|A|$W|X]]|$1|Y|3|Z|5|6|7|1F|8|@]|9|@]|A|$]]|$1|10|3|-4|5|6|7|1G|8|@]|9|@]|A|$]]]|11|$]]

First understand the structure of database.
<ul>
<li>Id is generated using sequence</li>
<li>increment done by 1 (specified while creating sequence)</li>
<li>Last entry to database will have highest value of <code>id</code></li>
</ul>
If you wanted to get <code>id</code> of record which is going to be saved,
Then you can use following:
<pre><code> 1. id = Model.last.id + 1
 model = Model.new(id: id)
 model.save
 # But if data can be delete from that dataabse this will not work correctly.
 
 2. id = Model.connection.select_value(&quot;Select nextval('models_id_seq')&quot;)
 model = Model.new(id: id)
 model.save
 # Here in this case if you did not specified 'id' while creating new object, record will saved with next id. 
 
 e.g. 
 id
 =&gt; 2234
 model = Model.new(id: id) 
 model.save 
 # Record will be created using 'id' as 2234 
 
 model = Model.new()
 model.save
 # Record will be created using next value of 'id' as 2235 
 
</code></pre>
Hope this will help you.

blocks|key|2715523|text|我刚刚在创建数据导入器时遇到了类似的情况。我创建了一堆不同类型的记录，并在保存之前将它们关联起来。在保存时，一些记录抛出了验证错误，因为它们有一个尚未保存的validate_presence_of记录。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2715524|如果您正在使用postgres，活动记录将通过保留名为models_id_seq的序列(sales_id_seq用于销售等)来递增其分配给模型的id。在数据库里。您可以获取此序列中的下一个id，并使用以下函数对其进行递增。|2715525|def+next_model_id
++++ActiveRecord::Base.connection.execute("SELECT+NEXTVAL('models_id_seq')").first["nextval"].to_i
end|code-block|syntax|javascript|2715526|然而，这种解决方案并不是很好的实践，因为不能保证活动记录将来会以这种方式保存id序列。我只会在我的项目中使用它一次，节省了我大量的工作，并且有很好的文档说明为什么不应该经常使用它。|2715527|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|O|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

I just ran into a similar situation when creating a data importer. I was creating a bunch of records of different types and associating them before saving. When saving, some of the records threw validation errors because they had validate_presence_of a record that was not yet saved.

If you are using postgres, active record increments the id it assigns to a Model by keeping a sequence named models_id_seq (sales_id_seq for Sale etc.) in the database. You can get the next id in this sequence and increment it with the following function.

<pre><code>def next_model_id
 ActiveRecord::Base.connection.execute("SELECT NEXTVAL('models_id_seq')").first["nextval"].to_i
end
</code></pre>

However, this solution is not good practice as there is no guarantee that active record will keep id sequences in this way in the future. I would only use this if it was used only once in my project, saved me a lot of work and was well documented in terms of why it should not be used frequently.

blocks|key|1623141|text|我知道这是一个老生常谈的问题，但最好还是放弃我的答案，以防有人需要参考它|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1623142|UserModel|offset|length|style|BOLD|1623143|class+User+<+ActiveRecord::Base
before_create+:set_default_value

def+set_default_value
+++self.value+%7C%7C=+"#{User.last.id%2B1}"
end|code-block|syntax|javascript|1623144|entityMap^0|0|0|9|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@$D|Q|E|R|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|S|8|@]|9|@]|A|$K|L]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

I know it's an old question, but might as well throw my answer in case anyone needs to reference it

UserModel

<pre><code>class User &lt; ActiveRecord::Base
before_create :set_default_value

def set_default_value
 self.value ||= "#{User.last.id+1}"
end
</code></pre>

How do you get the id of a rails model before it is saved?

For example, if I create a new model instance, how can I get its ID before it is saved? 

I know that the id is created onsave and according to the database but is there a workaround?

Get ID of Rails Model before saving...?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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在保存rails模型之前，如何获取它的id？例如，如果我创建了一个新的模型实例，我如何在保存它之前获得它的ID？我知道id是在保存时根据数据库创建的，但是有什么解决方法吗？

问保存前获取Rails模型的ID ...？
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回答 10

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问保存前获取Rails模型的ID ...？EN

回答 10

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问保存前获取Rails模型的ID ...？
EN