blocks|key|555347|text|时间戳帮助器仅在create_table块中可用。您可以通过手动指定列类型来添加这些列：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|555348|class+AddTimestampsToUser+<+ActiveRecord::Migration
++def+change_table
++++add_column+:users,+:created_at,+:datetime,+null:+false
++++add_column+:users,+:updated_at,+:datetime,+null:+false
++end
end|code-block|syntax|javascript|555349|虽然这与上面指定的add_timestamps方法没有相同的简洁语法，但Rails仍然会将这些列视为时间戳列，并正常更新值。|555350|entityMap^0|8|C|0|0|9|E|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|N|$]]

The timestamp helper is only available in the <code>create_table</code> block. You can add these columns by specifying the column types manually: 

<pre><code>class AddTimestampsToUser &lt; ActiveRecord::Migration
 def change_table
 add_column :users, :created_at, :datetime, null: false
 add_column :users, :updated_at, :datetime, null: false
 end
end
</code></pre>

While this does not have the same terse syntax as the <code>add_timestamps</code> method you have specified above, Rails will still treat these columns as timestamp columns, and update the values normally.

blocks|key|555436|text|迁移只是两个类方法(在3.1中是实例方法)：up和down+(在3.1中有时是change实例方法)。您希望您的更改进入up方法：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|555437|class+AddTimestampsToUser+<+ActiveRecord::Migration
++def+self.up+#+Or+`def+up`+in+3.1
++++change_table+:users+do+%7Ct%7C
++++++t.timestamps
++++end
++end
++def+self.down+#+Or+`def+down`+in+3.1
++++remove_column+:users,+:created_at
++++remove_column+:users,+:updated_at
++end
end|code-block|syntax|javascript|555438|如果你在3.1版本中，你也可以使用change+(谢谢戴夫)：|555439|class+AddTimestampsToUser+<+ActiveRecord::Migration
++def+change
++++change_table(:users)+{+%7Ct%7C+t.timestamps+}
++end
end|555440|也许您混淆了def+change、def+change_table和change_table。|555441|有关更多详细信息，请参阅migration+guide。|555442|entityMap|0|LINK|mutability|MUTABLE|url|http://guides.rubyonrails.org/migrations.html#special-helpers^0|M|2|P|4|13|6|1O|2|0|0|H|6|0|0|6|A|H|G|Y|C|0|C|F|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@$9|11|A|12|B|C]|$9|13|A|14|B|C]|$9|15|A|16|B|C]|$9|17|A|18|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|19|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|1A|8|@$9|1B|A|1C|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|1D|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|1E|8|@$9|1F|A|1G|B|C]|$9|1H|A|1I|B|C]|$9|1J|A|1K|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|6|7|1L|8|@]|D|@$9|1M|A|1N|1|1O]]|E|$]]|$1|S|3|-4|5|6|7|1P|8|@]|D|@]|E|$]]]|T|$U|$5|V|W|X|E|$Y|Z]]]]

Migrations are just two class methods (or instance methods in 3.1): <code>up</code> and <code>down</code> (and sometimes a <code>change</code> instance method in 3.1). You want your changes to go into the <code>up</code> method:

<pre><code>class AddTimestampsToUser &lt; ActiveRecord::Migration
 def self.up # Or `def up` in 3.1
 change_table :users do |t|
 t.timestamps
 end
 end
 def self.down # Or `def down` in 3.1
 remove_column :users, :created_at
 remove_column :users, :updated_at
 end
end
</code></pre>

If you're in 3.1 then you could also use <code>change</code> (thanks Dave):

<pre><code>class AddTimestampsToUser &lt; ActiveRecord::Migration
 def change
 change_table(:users) { |t| t.timestamps }
 end
end
</code></pre>

Perhaps you're confusing <code>def change</code>, <code>def change_table</code>, and <code>change_table</code>.

See the <a href="http://guides.rubyonrails.org/migrations.html#special-helpers" rel="noreferrer">migration guide</a> for further details.

blocks|key|555495|text|class+AddTimestampsToUser+<+ActiveRecord::Migration
++def+change
++++change_table+:users+do+%7Ct%7C
++++++t.timestamps
++++end
++end
end|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|555496|可用的转换包括|unstyled|555497|change_table+:table+do+%7Ct%7C
++t.column
++t.index
++t.timestamps
++t.change
++t.change_default
++t.rename
++t.references
++t.belongs_to
++t.string
++t.text
++t.integer
++t.float
++t.decimal
++t.datetime
++t.timestamp
++t.time
++t.date
++t.binary
++t.boolean
++t.remove
++t.remove_references
++t.remove_belongs_to
++t.remove_index
++t.remove_timestamps
end|555498|http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/Table.html|offset|length|555499|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|0|0|25|0|0^^$0|@$1|2|3|4|5|6|7|T|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|V|8|@]|9|@]|A|$B|C]]|$1|I|3|J|5|F|7|W|8|@]|9|@$K|X|L|Y|1|Z]]|A|$]]|$1|M|3|-4|5|F|7|10|8|@]|9|@]|A|$]]]|N|$O|$5|P|Q|R|A|$S|J]]]]

<pre><code>class AddTimestampsToUser &lt; ActiveRecord::Migration
 def change
 change_table :users do |t|
 t.timestamps
 end
 end
end
</code></pre>

Available transformations are

<pre><code>change_table :table do |t|
 t.column
 t.index
 t.timestamps
 t.change
 t.change_default
 t.rename
 t.references
 t.belongs_to
 t.string
 t.text
 t.integer
 t.float
 t.decimal
 t.datetime
 t.timestamp
 t.time
 t.date
 t.binary
 t.boolean
 t.remove
 t.remove_references
 t.remove_belongs_to
 t.remove_index
 t.remove_timestamps
end
</code></pre>

<a href="http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/Table.html" rel="noreferrer">http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/Table.html</a>

blocks|key|555548|text|您的原始代码非常接近正确，您只需要使用不同的方法名称。如果您使用的是Rails3.1或更高版本，则需要定义change方法而不是change_table方法|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|555549|class+AddTimestampsToUser+<+ActiveRecord::Migration
++def+change
++++add_timestamps(:users)
++end
end|code-block|syntax|javascript|555550|如果您使用的是旧版本，则需要定义up和down方法，而不是change_table|555551|class+AddTimestampsToUser+<+ActiveRecord::Migration
++def+up
++++add_timestamps(:users)
++end

++def+down
++++remove_timestamps(:users)
++end
end|555552|entityMap^0|1H|6|1S|C|0|0|G|2|J|4|T|C|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|V|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|13|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|14|8|@]|D|@]|E|$]]]|P|$]]

Your original code is very close to right, you just need to use a different method name. If you're using Rails 3.1 or later, you need to define a <code>change</code> method instead of <code>change_table</code>:

<pre><code>class AddTimestampsToUser &lt; ActiveRecord::Migration
 def change
 add_timestamps(:users)
 end
end
</code></pre>

If you're using an older version you need to define <code>up</code> and <code>down</code> methods instead of <code>change_table</code>:

<pre><code>class AddTimestampsToUser &lt; ActiveRecord::Migration
 def up
 add_timestamps(:users)
 end

 def down
 remove_timestamps(:users)
 end
end
</code></pre>

blocks|key|772086|text|对于那些不使用Rails但使用activerecord的人，下面的代码还会在现有模型中添加一列，例如整型字段。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|772087|ActiveRecord::Schema.define+do
++change_table+'MYTABLE'+do+%7Ctable%7C
++++add_column(:mytable,+:my_field_name,+:integer)
++end
end|code-block|syntax|javascript|772088|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|$]]

For those who don't use Rails but do use activerecord, the following also adds a column to an existing model, example is for an integer field.

<pre><code>ActiveRecord::Schema.define do
 change_table 'MYTABLE' do |table|
 add_column(:mytable, :my_field_name, :integer)
 end
end
</code></pre>

blocks|key|555656|text|前面的答案似乎是正确的，但是如果我的表已经有条目，我就会遇到问题。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|555657|我会得到“ERROR:+column+created_at+contains+null+values”。|offset|length|style|CODE|555658|为了解决这个问题，我使用：|555659|def+up
++add_column+:projects,+:created_at,+:datetime,+default:+nil,+null:+false
++add_column+:projects,+:updated_at,+:datetime,+default:+nil,+null:+false
end|code-block|syntax|javascript|555660|然后，我使用gem+migration_data为当前项目添加迁移时间，例如：|555661|def+data
++Project.update_all+created_at:+Time.now
end|555662|则迁移后创建的所有项目都将正确更新。确保服务器也重新启动，以便Rails+ActiveRecord开始跟踪记录上的时间戳。|555663|entityMap|0|LINK|mutability|MUTABLE|url|https://github.com/ka8725/migration_data^0|0|J|A|13|4|0|0|0|A|E|0|0|0|11|C|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|13|8|@$D|14|E|15|F|G]|$D|16|E|17|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|18|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|19|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|1A|8|@]|9|@$D|1B|E|1C|1|1D]]|A|$]]|$1|Q|3|R|5|L|7|1E|8|@]|9|@]|A|$M|N]]|$1|S|3|T|5|6|7|1F|8|@$D|1G|E|1H|F|G]]|9|@]|A|$]]|$1|U|3|-4|5|6|7|1I|8|@]|9|@]|A|$]]]|V|$W|$5|X|Y|Z|A|$10|11]]]]

The answers before seem right however I faced issues if my table already has entries.

I would get 'ERROR: column <code>created_at</code> contains <code>null</code> values'.

To fix, I used:

<pre><code>def up
 add_column :projects, :created_at, :datetime, default: nil, null: false
 add_column :projects, :updated_at, :datetime, default: nil, null: false
end
</code></pre>

I then used the gem <a href="https://github.com/ka8725/migration_data" rel="nofollow noreferrer">migration_data</a> to add the time for current projects on the migration such as:

<pre><code>def data
 Project.update_all created_at: Time.now
end
</code></pre>

Then all projects created after this migration will be correctly updated. Make sure the server is restarted too so that Rails <code>ActiveRecord</code> starts tracking the timestamps on the record.

blocks|key|772152|text|def+change
++add_timestamps+:table_name
end|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|772153|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>def change
 add_timestamps :table_name
end
</code></pre>

blocks|key|772214|text|@user1899434的回应是，这里的“现有”表可能意味着其中已经有记录的表，这些记录可能是您不想删除的记录。因此，当您添加带有null:+false的时间戳时，这些现有记录都是无效的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|772215|但我认为，通过将两个步骤结合到一个迁移中，以及使用更具语义的add_timestamps方法，可以改进这个答案：|772216|def+change
++add_timestamps+:projects,+default:+Time.zone.now
++change_column_default+:projects,+:created_at,+nil
++change_column_default+:projects,+:updated_at,+nil
end|code-block|syntax|javascript|772217|您可以用其他时间戳替换DateTime.now，例如，如果您希望在黎明时创建/更新预先存在的记录。|offset|length|style|CODE|772218|entityMap^0|0|0|0|B|C|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|T|8|@$K|U|L|V|M|N]]|9|@]|A|$]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

@user1899434's response picked up on the fact that an "existing" table here could mean a table with records already in it, records that you might not want to drop. So when you add timestamps with null: false, which is the default and often desirable, those existing records are all invalid.

But I think that answer can be improved upon, by combining the two steps into one migration, as well as using the more semantic add_timestamps method:

<pre><code>def change
 add_timestamps :projects, default: Time.zone.now
 change_column_default :projects, :created_at, nil
 change_column_default :projects, :updated_at, nil
end
</code></pre>

You could substitute some other timestamp for <code>DateTime.now</code>, like if you wanted preexisting records to be created/updated at the dawn of time instead.

blocks|key|555775|text|我个人使用了以下内容，它用当前时间/日期更新了所有以前的记录：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|555776|add_column+:<table>,+:created_at,+:datetime,+default:+Time.zone.now,+null:+false
add_column+:<table>,+:updated_at,+:datetime,+default:+Time.zone.now,+null:+false|code-block|syntax|javascript|555777|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|$]]

I personally used the following, and it updated all previous records with the current time/date:

<pre><code>add_column :&lt;table&gt;, :created_at, :datetime, default: Time.zone.now, null: false
add_column :&lt;table&gt;, :updated_at, :datetime, default: Time.zone.now, null: false
</code></pre>

blocks|key|772290|text|这是change，而不是用于Rails4.2的change_table：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|772291|class+AddTimestampsToUsers+<+ActiveRecord::Migration
++def+change
++++add_timestamps(:users)
++end
end|code-block|syntax|javascript|772292|entityMap^0|2|6|N|C|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|L|$]]

It's <code>change</code>, not <code>change_table</code> for Rails 4.2:

<pre><code>class AddTimestampsToUsers &lt; ActiveRecord::Migration
 def change
 add_timestamps(:users)
 end
end
</code></pre>

blocks|key|772396|text|就向具有现有数据的表中添加时间戳列而言，Nick+Davies+answer是最完整的。它唯一的缺点是它将在db:rollback上引发ActiveRecord::IrreversibleMigration。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|772397|应该像这样修改它，以便在两个方向上都能工作：|772398|def+change
++add_timestamps+:campaigns,+default:+DateTime.now
++change_column_default+:campaigns,+:created_at,+from:+DateTime.now,+to:+nil
++change_column_default+:campaigns,+:updated_at,+from:+DateTime.now,+to:+nil
end|code-block|syntax|javascript|772399|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/43548750/111363^0|1I|B|1W|Z|K|I|0|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]]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|F|3|G|5|6|7|12|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|13|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|14|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]]]

<a href="https://stackoverflow.com/a/43548750/111363">Nick Davies answer</a> is the most complete in terms of adding timestamp columns to a table with existing data. Its only downside is that it will raise <code>ActiveRecord::IrreversibleMigration</code> on a <code>db:rollback</code>.

It should be modified like so to work in both directions:

<pre><code>def change
 add_timestamps :campaigns, default: DateTime.now
 change_column_default :campaigns, :created_at, from: DateTime.now, to: nil
 change_column_default :campaigns, :updated_at, from: DateTime.now, to: nil
end
</code></pre>

blocks|key|772440|text|不确定这是什么时候引入的，但在rails+5.2.1中，您可以这样做：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|772441|class+AddTimestampsToMyTable+<+ActiveRecord::Migration[5.2]
++def+change
++++add_timestamps+:my_table
++end
end|code-block|syntax|javascript|772442|有关详细信息，请参阅活动记录迁移文档中的"using+the+change+method“。|offset|length|772443|entityMap|0|LINK|mutability|MUTABLE|url|https://edgeguides.rubyonrails.org/active_record_migrations.html#using-the-change-method^0|0|0|L|N|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]]|$1|G|3|H|5|6|7|U|8|@]|9|@$I|V|J|W|1|X]]|A|$]]|$1|K|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|L|$M|$5|N|O|P|A|$Q|R]]]]

not sure when exactly this was introduced, but in rails 5.2.1 you can do this:

<pre><code>class AddTimestampsToMyTable &lt; ActiveRecord::Migration[5.2]
 def change
 add_timestamps :my_table
 end
end
</code></pre>

for more see "<a href="https://edgeguides.rubyonrails.org/active_record_migrations.html#using-the-change-method" rel="nofollow noreferrer">using the change method</a>" in the active record migrations docs.

blocks|key|556013|text|这里有很多答案，但我也会发布我的答案，因为之前的答案对我来说都不是很有效:)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|556014|正如一些人所指出的，不幸的是，#add_timestamps添加了null:+false限制，这将导致旧的行无效，因为它们没有填充这些值。这里的大多数答案建议我们设置一些默认值(Time.zone.now)，但我不想这样做，因为这些旧数据的默认时间戳将不正确。我看不出向表中添加不正确的数据有什么价值。|offset|length|style|CODE|556015|所以我的迁移很简单：|556016|class+AddTimestampsToUser+<+ActiveRecord::Migration
++def+change_table
++++add_column+:projects,+:created_at,+:datetime
++++add_column+:projects,+:updated_at,+:datetime
++end
end|code-block|syntax|javascript|556017|没有null:+false，没有其他限制。当created_at作为NULL，update_at作为NULL时，旧的行将继续有效(直到对行执行某些更新为止)。新行将按预期填充created_at和updated_at。|556018|entityMap^0|0|F|F|X|B|2H|D|0|0|0|2|B|M|A|Y|4|13|9|1E|4|2F|A|2Q|A|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@$D|U|E|V|F|G]|$D|W|E|X|F|G]|$D|Y|E|Z|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|10|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|11|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|12|8|@$D|13|E|14|F|G]|$D|15|E|16|F|G]|$D|17|E|18|F|G]|$D|19|E|1A|F|G]|$D|1B|E|1C|F|G]|$D|1D|E|1E|F|G]|$D|1F|E|1G|F|G]]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|1H|8|@]|9|@]|A|$]]]|R|$]]

A lot of answers here, but I'll post mine too because none of the previous ones really worked for me :) 

As some have noted, <code>#add_timestamps</code> unfortunately adds the <code>null: false</code> restriction, which will cause old rows to be invalid because they don't have these values populated. Most answers here suggest that we set some default value (<code>Time.zone.now</code>), but I wouldn't like to do that because these default timestamps for old data will not be correct. I don't see the value in adding incorrect data to the table.

So my migration was simply: 

<pre><code>class AddTimestampsToUser &lt; ActiveRecord::Migration
 def change_table
 add_column :projects, :created_at, :datetime
 add_column :projects, :updated_at, :datetime
 end
end
</code></pre>

No <code>null: false</code>, no other restrictions. Old rows will continue being valid with <code>created_at</code> as <code>NULL</code>, and <code>update_at</code> as <code>NULL</code> (until some update is performed to the row). New rows will have <code>created_at</code> and <code>updated_at</code> populated as expected.

blocks|key|772564|text|我在Rails5上遇到了同样的问题|type|unstyled|depth|inlineStyleRanges|entityRanges|data|772565|change_table+:my_table+do+%7Ct%7C
++++t.timestamps
end|code-block|syntax|javascript|772566|我可以使用以下内容手动添加时间戳列：|772567|change_table+:my_table+do+%7Ct%7C
++++t.datetime+:created_at,+null:+false,+default:+DateTime.now
++++t.datetime+:updated_at,+null:+false,+default:+DateTime.now
end|772568|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

I ran into the same issue on Rails 5 trying to use 

<pre><code>change_table :my_table do |t|
 t.timestamps
end
</code></pre>

I was able to add the timestamp columns manually with the following:

<pre><code>change_table :my_table do |t|
 t.datetime :created_at, null: false, default: DateTime.now
 t.datetime :updated_at, null: false, default: DateTime.now
end
</code></pre>

blocks|key|556166|text|这里的大多数答案的问题是，如果您默认使用Time.zone.now，则所有记录都将运行迁移的时间作为其默认时间，这可能不是您想要的。在Rails5中，您可以使用now()。这将现有记录的时间戳设置为运行迁移的时间，以及新插入记录的提交事务的开始时间。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|556167|class+AddTimestampsToUsers+<+ActiveRecord::Migration+++def+change+++++add_timestamps+:users,+default:+->+{+'now()'+},+null:+false+++end+end|556168|entityMap^0|K|D|28|5|0|0|3V|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|R|8|@]|D|@]|E|$]]]|I|$]]

The issue with most of the answers here is that if you default to <code>Time.zone.now</code> all records will have the time that the migration was run as their default time, which is probably not what you want. In rails 5 you can instead use <code>now()</code>. This will set the timestamps for existing records as the time the migration was run, and as the start time of the commit transaction for newly inserted records.

<code>class AddTimestampsToUsers &lt; ActiveRecord::Migration
 def change
 add_timestamps :users, default: -&gt; { 'now()' }, null: false
 end
end
</code>

blocks|key|772682|text|使用Time.current是一种很好的https://github.com/rubocop-hq/rails-style-guide#timenow风格|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|772683|def+change
++change_table+:users+do+%7Ct%7C
++++t.timestamps+default:+Time.current
++++t.change_default+:created_at,+from:+Time.current,+to:+nil
++++t.change_default+:updated_at,+from:+Time.current,+to:+nil
++end
end|code-block|syntax|javascript|772684|或|772685|def+change
++add_timestamps+:users,+default:+Time.current
++change_column_default+:users,+:created_at,+from:+Time.current,+to:+nil
++change_column_default+:users,+:updated_at,+from:+Time.current,+to:+nil
end|772686|entityMap|0|LINK|mutability|MUTABLE|url|https://github.com/rubocop-hq/rails-style-guide#timenow^0|2|C|K|1J|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|F|3|G|5|H|7|12|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|13|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|14|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|15|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]]]

Using <code>Time.current</code> is a good style <a href="https://github.com/rubocop-hq/rails-style-guide#timenow" rel="noreferrer">https://github.com/rubocop-hq/rails-style-guide#timenow</a>

<pre><code>def change
 change_table :users do |t|
 t.timestamps default: Time.current
 t.change_default :created_at, from: Time.current, to: nil
 t.change_default :updated_at, from: Time.current, to: nil
 end
end
</code></pre>

or

<pre><code>def change
 add_timestamps :users, default: Time.current
 change_column_default :users, :created_at, from: Time.current, to: nil
 change_column_default :users, :updated_at, from: Time.current, to: nil
end
</code></pre>

blocks|key|772725|text|这看起来像是Rails+5.0.7中的一个干净的解决方案(发现了change_column_null方法)：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|772726|def+change
++add_timestamps+:candidate_offices,+default:+nil,+null:+true
++change_column_null(:candidate_offices,+:created_at,+false,+Time.zone.now)
++change_column_null(:candidate_offices,+:created_at,+false,+Time.zone.now)
end|code-block|syntax|javascript|772727|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|$]]

This seems like a clean solution in Rails 5.0.7 (discovered the change_column_null method):

<pre><code>def change
 add_timestamps :candidate_offices, default: nil, null: true
 change_column_null(:candidate_offices, :created_at, false, Time.zone.now)
 change_column_null(:candidate_offices, :created_at, false, Time.zone.now)
end
</code></pre>

blocks|key|556294|text|我使用的是rails+5.0，这些选项都不起作用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|556295|唯一有效的方法是将类型设置为:timestamp而不是:datetime|556296|def+change
++++add_column+:users,+:created_at,+:timestamp
++++add_column+:users,+:updated_at,+:timestamp
end|code-block|syntax|javascript|556297|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|$]]

I'm on rails 5.0 and none of these options worked.

The only thing that worked was using the type to be :timestamp and not :datetime

<pre><code>def change
 add_column :users, :created_at, :timestamp
 add_column :users, :updated_at, :timestamp
end
</code></pre>

I need to add timestamps (<code>created_at</code> &amp; <code>updated_at</code>) to an existing table. I tried the following code but it didn't work.

<pre><code>class AddTimestampsToUser &lt; ActiveRecord::Migration
 def change_table
 add_timestamps(:users)
 end
end
</code></pre>

Add timestamps to an existing table

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我需要将时间戳(created_at & updated_at)添加到现有表中。我尝试了下面的代码，但它不起作用。class AddTimestampsToUser < ActiveRecord::Migration def change_table add_timestamps(:users) ...

问向现有表添加时间戳
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