blocks|key|902528|text|创建具有带:default+=>值的change_column语句的迁移。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|902529|change_column+:my_table,+:my_column,+:integer,+:default+=>+0,+:null+=>+false|code-block|syntax|javascript|902530|请参阅：change_column|902531|根据数据库引擎的不同，您可能需要使用change_column_null|902532|entityMap|0|LINK|mutability|MUTABLE|url|http://apidock.com/rails/ActiveRecord/ConnectionAdapters/SchemaStatements/change_column^0|5|B|I|D|0|0|4|D|0|0|I|I|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|12|8|@]|D|@$9|13|A|14|1|15]]|E|$]]|$1|M|3|N|5|6|7|16|8|@$9|17|A|18|B|C]]|D|@]|E|$]]|$1|O|3|-4|5|6|7|19|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]]]

Create a migration that has a <code>change_column</code> statement with a <code>:default =&gt;</code> value.

<pre><code>change_column :my_table, :my_column, :integer, :default =&gt; 0, :null =&gt; false
</code></pre>

See: <a href="http://apidock.com/rails/ActiveRecord/ConnectionAdapters/SchemaStatements/change_column" rel="noreferrer">change_column</a>

Depending on the database engine you may need to use <code>change_column_null</code>

blocks|key|686055|text|如果您在迁移中这样做，那么您可能会这样做：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|686056|#+Make+sure+no+null+value+exist
MyModel.where(date_column:+nil).update_all(date_column:+Time.now)

#+Change+the+column+to+not+allow+null
change_column+:my_models,+:date_column,+:datetime,+null:+false|code-block|syntax|javascript|686057|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|$]]

If you do it in a migration then you could probably do it like this:

<pre><code># Make sure no null value exist
MyModel.where(date_column: nil).update_all(date_column: Time.now)

# Change the column to not allow null
change_column :my_models, :date_column, :datetime, null: false
</code></pre>

blocks|key|902665|text|在Rails+4中，这是一个更好的(DRYer)解决方案：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|902666|change_column_null+:my_models,+:date_column,+false|code-block|syntax|javascript|902667|要确保该列中不存在具有NULL值的记录，可以传递第四个参数，该参数是用于具有NULL值的记录的默认值：|offset|length|style|CODE|902668|change_column_null+:my_models,+:date_column,+false,+Time.now|902669|entityMap^0|0|0|B|4|12|4|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@$I|T|J|U|K|L]|$I|V|J|W|K|L]]|9|@]|A|$]]|$1|M|3|N|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|P|$]]

In Rails 4, this is a better (DRYer) solution:

<pre><code>change_column_null :my_models, :date_column, false
</code></pre>

To ensure no records exist with <code>NULL</code> values in that column, you can pass a fourth parameter, which is the default value to use for records with <code>NULL</code> values:

<pre><code>change_column_null :my_models, :date_column, false, Time.now
</code></pre>

blocks|key|902679|text|Rails+4：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|902680|def+change
++change_column_null(:users,+:admin,+false+)
end|code-block|syntax|javascript|902681|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 4: 

<pre><code>def change
 change_column_null(:users, :admin, false )
end
</code></pre>

blocks|key|686211|text|如果您已有记录，则不能使用add_timestamps和null:false，因此以下是解决方案：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|686212|def+change
++add_timestamps(:buttons,+null:+true)

++Button.find_each+{+%7Cb%7C+b.update(created_at:+Time.zone.now,+updated_at:+Time.zone.now)+}

++change_column_null(:buttons,+:created_at,+false)
++change_column_null(:buttons,+:updated_at,+false)
end|code-block|syntax|javascript|686213|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|$]]

You can't use add_timestamps and null:false if you have existing records, so here is the solution :

<pre><code>def change
 add_timestamps(:buttons, null: true)

 Button.find_each { |b| b.update(created_at: Time.zone.now, updated_at: Time.zone.now) }

 change_column_null(:buttons, :created_at, false)
 change_column_null(:buttons, :updated_at, false)
end
</code></pre>

blocks|key|686285|text|根据Strong+Migrations+gem，在生产中使用change_column_null不是一个好主意，因为它会在检查所有记录的同时阻塞读写。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|686286|处理这些迁移(特定于Postgres)的推荐方法是将此过程分成两个迁移。|686287|一个用来修改带有约束的表：|686288|class+SetSomeColumnNotNull+<+ActiveRecord::Migration[6.0]
++def+change
++++safety_assured+do
++++++execute+'ALTER+TABLE+"users"+ADD+CONSTRAINT+"users_some_column_null"+CHECK+("some_column"+IS+NOT+NULL)+NOT+VALID'
++++end
++end
end|code-block|syntax|javascript|686289|和一个单独的迁移来验证它：|686290|class+ValidateSomeColumnNotNull+<+ActiveRecord::Migration[6.0]
++def+change
++++safety_assured+do
++++++execute+'ALTER+TABLE+"users"+VALIDATE+CONSTRAINT+"users_some_column_null"'
++++end
++end
end|686291|上面的例子是从链接的文档中提取出来的(并稍作修改)。显然，对于Postgres+12%2B，您还可以将NOT+NULL添加到模式中，然后在运行验证后删除约束：|686292|class+ValidateSomeColumnNotNull+<+ActiveRecord::Migration[6.0]
++def+change
++++safety_assured+do
++++++execute+'ALTER+TABLE+"users"+VALIDATE+CONSTRAINT+"users_some_column_null"'
++++end

++++#+in+Postgres+12%2B,+you+can+then+safely+set+NOT+NULL+on+the+column
++++change_column_null+:users,+:some_column,+false
++++safety_assured+do
++++++execute+'ALTER+TABLE+"users"+DROP+CONSTRAINT+"users_some_column_null"'
++++end
++end
end|686293|当然，这意味着对于Postgres的早期版本，您的模式不会显示该列为NOT+NULL，因此我还建议设置一个模型级别的验证，以要求提供该值(尽管我建议即使对于允许此步骤的PG版本也是如此)。|686294|此外，在运行这些迁移之前，您需要用null以外的值更新所有现有记录，并确保写入表的任何生产代码都没有为值写入null。|686295|entityMap|0|LINK|mutability|MUTABLE|url|https://github.com/ankane/strong_migrations#setting-not-null-on-an-existing-column^0|U|I|2|L|0|0|0|0|0|0|0|1D|8|0|0|Y|8|0|1I|4|0^^$0|@$1|2|3|4|5|6|7|18|8|@$9|19|A|1A|B|C]]|D|@$9|1B|A|1C|1|1D]]|E|$]]|$1|F|3|G|5|6|7|1E|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|1F|8|@]|D|@]|E|$]]|$1|J|3|K|5|L|7|1G|8|@]|D|@]|E|$M|N]]|$1|O|3|P|5|6|7|1H|8|@]|D|@]|E|$]]|$1|Q|3|R|5|L|7|1I|8|@]|D|@]|E|$M|N]]|$1|S|3|T|5|6|7|1J|8|@$9|1K|A|1L|B|C]]|D|@]|E|$]]|$1|U|3|V|5|L|7|1M|8|@]|D|@]|E|$M|N]]|$1|W|3|X|5|6|7|1N|8|@$9|1O|A|1P|B|C]]|D|@]|E|$]]|$1|Y|3|Z|5|6|7|1Q|8|@$9|1R|A|1S|B|C]]|D|@]|E|$]]|$1|10|3|-4|5|6|7|1T|8|@]|D|@]|E|$]]]|11|$12|$5|13|14|15|E|$16|17]]]]

Per the <a href="https://github.com/ankane/strong_migrations#setting-not-null-on-an-existing-column" rel="nofollow noreferrer">Strong Migrations gem</a>, using <code>change_column_null</code> in production is a bad idea because it blocks reads and writes while all records are checked.
The recommended way to handle these migrations (Postgres specific) is to separate this process into two migrations.
One to alter the table with the constraint:
<pre><code>class SetSomeColumnNotNull &lt; ActiveRecord::Migration[6.0]
 def change
 safety_assured do
 execute 'ALTER TABLE &quot;users&quot; ADD CONSTRAINT &quot;users_some_column_null&quot; CHECK (&quot;some_column&quot; IS NOT NULL) NOT VALID'
 end
 end
end
</code></pre>
And a separate migration to validate it:
<pre><code>class ValidateSomeColumnNotNull &lt; ActiveRecord::Migration[6.0]
 def change
 safety_assured do
 execute 'ALTER TABLE &quot;users&quot; VALIDATE CONSTRAINT &quot;users_some_column_null&quot;'
 end
 end
end
</code></pre>
The above examples are pulled (and slightly altered) from the linked documentation. Apparently for Postgres 12+ you can also add <code>NOT NULL</code> to the schema and then drop the constraint after the validation has been run:
<pre><code>class ValidateSomeColumnNotNull &lt; ActiveRecord::Migration[6.0]
 def change
 safety_assured do
 execute 'ALTER TABLE &quot;users&quot; VALIDATE CONSTRAINT &quot;users_some_column_null&quot;'
 end

 # in Postgres 12+, you can then safely set NOT NULL on the column
 change_column_null :users, :some_column, false
 safety_assured do
 execute 'ALTER TABLE &quot;users&quot; DROP CONSTRAINT &quot;users_some_column_null&quot;'
 end
 end
end
</code></pre>
Naturally, this means your schema will not show that the column is <code>NOT NULL</code> for earlier versions of Postgres, so I'd also advise setting a model level validation to require the value to be present (though I'd suggest the same even for versions of PG that do allow this step).
Further, before running these migrations you'll want to update all existing records with a value other than null, and make sure any production code that writes to the table is not writing <code>null</code> for the value(s).

I created a date column in a previous migration and set it to be nullable. Now I want to change it to be not nullable. How do I go about doing this assuming there are null rows in that database? I'm ok with setting those columns to Time.now if they're currently null.

How to change a nullable column to not nullable in a Rails migration?

数据库

我在之前的迁移中创建了一个日期列，并将其设置为可空。现在我想把它改成不可为空的。假设数据库中有空行，我该如何执行此操作？如果这些列当前为空，我可以将它们设置为Time.now。

问如何在Rails迁移中将可空的列更改为不可空的？
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