blocks|key|481464|text|change_column是ActiveRecord::Migration的一个方法，所以你不能在控制台这样调用它。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|481465|如果要为该列添加默认值，请创建新迁移：|481466|rails+g+migration+add_default_value_to_show_attribute|481467|然后在创建的迁移中：|481468|#+That's+the+more+generic+way+to+change+a+column
def+up
++change_column+:profiles,+:show_attribute,+:boolean,+default:+true
end

def+down
++change_column+:profiles,+:show_attribute,+:boolean,+default:+nil
end|code-block|syntax|javascript|481469|或者更具体的选项：|481470|def+up
++++change_column_default+:profiles,+:show_attribute,+true
end

def+down
++++change_column_default+:profiles,+:show_attribute,+nil
end|481471|然后运行rake+db:migrate。|481472|它不会对已创建的记录进行任何更改。要做到这一点，您必须创建一个rake+task，或者直接在rails+console中更新所有记录(我不建议在生产环境中这样做)。|481473|当您将t.boolean+:show_attribute,+:default+=>+true添加到create_profiles迁移中时，预计它不会执行任何操作。仅执行尚未运行的迁移。如果您从一个新的数据库开始，那么它会将缺省值设置为true。|481474|entityMap^0|0|D|E|N|0|0|0|1H|0|0|0|0|0|4|F|0|V|9|1A|D|0|3|17|1D|F|0^^$0|@$1|2|3|4|5|6|7|12|8|@$9|13|A|14|B|C]|$9|15|A|16|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|17|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|18|8|@$9|19|A|1A|B|C]]|D|@]|E|$]]|$1|J|3|K|5|6|7|1B|8|@]|D|@]|E|$]]|$1|L|3|M|5|N|7|1C|8|@]|D|@]|E|$O|P]]|$1|Q|3|R|5|6|7|1D|8|@]|D|@]|E|$]]|$1|S|3|T|5|N|7|1E|8|@]|D|@]|E|$O|P]]|$1|U|3|V|5|6|7|1F|8|@$9|1G|A|1H|B|C]]|D|@]|E|$]]|$1|W|3|X|5|6|7|1I|8|@$9|1J|A|1K|B|C]|$9|1L|A|1M|B|C]]|D|@]|E|$]]|$1|Y|3|Z|5|6|7|1N|8|@$9|1O|A|1P|B|C]|$9|1Q|A|1R|B|C]]|D|@]|E|$]]|$1|10|3|-4|5|6|7|1S|8|@]|D|@]|E|$]]]|11|$]]

<code>change_column</code> is a method of <code>ActiveRecord::Migration</code>, so you can't call it like that in the console.
If you want to add a default value for this column, create a new migration:
<code>rails g migration add_default_value_to_show_attribute</code>
Then in the migration created:
<pre><code># That's the more generic way to change a column
def up
 change_column :profiles, :show_attribute, :boolean, default: true
end

def down
 change_column :profiles, :show_attribute, :boolean, default: nil
end
</code></pre>
OR a more specific option:
<pre><code>def up
 change_column_default :profiles, :show_attribute, true
end

def down
 change_column_default :profiles, :show_attribute, nil
end
</code></pre>
Then run <code>rake db:migrate</code>.
It won't change anything to the already created records. To do that you would have to create a <code>rake task</code> or just go in the <code>rails console</code> and update all the records (which I would not recommend in production).
When you added <code>t.boolean :show_attribute, :default =&gt; true</code> to the <code>create_profiles</code> migration, it's expected that it didn't do anything. Only migrations that have not already been ran are executed. If you started with a fresh database, then it would set the default to true.

blocks|key|697396|text|作为公认答案的变体，您还可以在迁移中使用change_column_default方法：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|697397|def+up
++change_column_default+:profiles,+:show_attribute,+true
end

def+down
++change_column_default+:profiles,+:show_attribute,+nil
end|code-block|syntax|javascript|697398|Rails+API-docs|697399|entityMap|0|LINK|mutability|MUTABLE|url|http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-change_column_default^0|K|L|0|0|0|E|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|Y|8|@]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|M|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]]]

As a variation on the accepted answer you could also use the <code>change_column_default</code> method in your migrations:

<pre><code>def up
 change_column_default :profiles, :show_attribute, true
end

def down
 change_column_default :profiles, :show_attribute, nil
end
</code></pre>

<a href="http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-change_column_default" rel="noreferrer">Rails API-docs</a>

blocks|key|697507|text|我不确定这是什么时候写的，但目前要在迁移中添加或删除列中的默认值，您可以使用以下命令：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|697508|change_column_null+:products,+:name,+false|code-block|syntax|javascript|697509|Rails+5：|697510|change_column_default+:products,+:approved,+from:+true,+to:+false|697511|http://edgeguides.rubyonrails.org/active_record_migrations.html#changing-columns|offset|length|697512|Rails+4.2：|697513|change_column_default+:products,+:approved,+false|697514|http://guides.rubyonrails.org/v4.2/active_record_migrations.html#changing-columns|697515|这是避免在迁移或模式中查找列规范的一种巧妙方法。|697516|entityMap|0|LINK|mutability|MUTABLE|url|1^0|0|0|0|0|0|28|0|0|0|0|0|29|1|0|0^^$0|@$1|2|3|4|5|6|7|14|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|15|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|16|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|17|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|18|8|@]|9|@$M|19|N|1A|1|1B]]|A|$]]|$1|O|3|P|5|6|7|1C|8|@]|9|@]|A|$]]|$1|Q|3|R|5|D|7|1D|8|@]|9|@]|A|$E|F]]|$1|S|3|T|5|6|7|1E|8|@]|9|@$M|1F|N|1G|1|1H]]|A|$]]|$1|U|3|V|5|6|7|1I|8|@]|9|@]|A|$]]|$1|W|3|-4|5|6|7|1J|8|@]|9|@]|A|$]]]|X|$Y|$5|Z|10|11|A|$12|L]]|13|$5|Z|10|11|A|$12|T]]]]

I'm not sure when this was written, but currently to add or remove a default from a column in a migration, you can use the following:

<pre><code>change_column_null :products, :name, false
</code></pre>

Rails 5:

<pre><code>change_column_default :products, :approved, from: true, to: false
</code></pre>

<a href="http://edgeguides.rubyonrails.org/active_record_migrations.html#changing-columns" rel="noreferrer">http://edgeguides.rubyonrails.org/active_record_migrations.html#changing-columns</a>

Rails 4.2: 

<pre><code>change_column_default :products, :approved, false
</code></pre>

<a href="http://guides.rubyonrails.org/v4.2/active_record_migrations.html#changing-columns" rel="noreferrer">http://guides.rubyonrails.org/v4.2/active_record_migrations.html#changing-columns</a>

Which is a neat way of avoiding looking through your migrations or schema for the column specifications.

blocks|key|697569|text|change_column+:things,+:price_1,+:integer,+default:+123,+null:+false|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|697570|这似乎是向还没有null:+false的现有列添加默认值的最佳方式。|unstyled|offset|length|style|CODE|697571|否则：|697572|change_column+:things,+:price_1,+:integer,+default:+123|697573|我在这方面做了一些研究：|697574|https://gist.github.com/Dorian/417b9a0e1a4e09a558c39345d50c8c3b|697575|entityMap|0|LINK|mutability|MUTABLE|url^0|0|8|B|0|0|0|0|0|1R|0|0^^$0|@$1|2|3|4|5|6|7|Z|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|10|8|@$G|11|H|12|I|J]]|9|@]|A|$]]|$1|K|3|L|5|F|7|13|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|14|8|@]|9|@]|A|$B|C]]|$1|O|3|P|5|F|7|15|8|@]|9|@]|A|$]]|$1|Q|3|R|5|F|7|16|8|@]|9|@$G|17|H|18|1|19]]|A|$]]|$1|S|3|-4|5|F|7|1A|8|@]|9|@]|A|$]]]|T|$U|$5|V|W|X|A|$Y|R]]]]

<pre><code>change_column :things, :price_1, :integer, default: 123, null: false
</code></pre>

Seems to be best way to add a default to an existing column that doesn't have <code>null: false</code> already.

Otherwise:

<pre><code>change_column :things, :price_1, :integer, default: 123
</code></pre>

Some research I did on this:

<a href="https://gist.github.com/Dorian/417b9a0e1a4e09a558c39345d50c8c3b" rel="nofollow noreferrer">https://gist.github.com/Dorian/417b9a0e1a4e09a558c39345d50c8c3b</a>

blocks|key|697631|text|如果您刚刚进行了迁移，则可以回滚，然后再次进行迁移。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|697632|要回滚，您可以执行任意多个步骤：|697633|rake+db:rollback+STEP=1|code-block|syntax|javascript|697634|或者，如果您使用的是Rails+5.2或更高版本：|697635|rails+db:rollback+STEP=1|697636|然后，您可以再次进行迁移：|697637|def+change
++add_column+:profiles,+:show_attribute,+:boolean,+default:+true
end|697638|如果你使用的是heroku+heroku+run+rake+db:migrate，别忘了使用rake+db:migrate|offset|length|style|CODE|697639|entityMap^0|0|0|0|0|0|0|0|E|Q|1A|F|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Z|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|10|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|11|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|12|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|13|8|@]|9|@]|A|$]]|$1|O|3|P|5|F|7|14|8|@]|9|@]|A|$G|H]]|$1|Q|3|R|5|6|7|15|8|@$S|16|T|17|U|V]|$S|18|T|19|U|V]]|9|@]|A|$]]|$1|W|3|-4|5|6|7|1A|8|@]|9|@]|A|$]]]|X|$]]

If you just made a migration, you can rollback and then make your migration again.

To rollback you can do as many steps as you want:

<pre><code>rake db:rollback STEP=1
</code></pre>

Or, if you are using Rails 5.2 or newer:

<pre><code>rails db:rollback STEP=1
</code></pre>

Then, you can just make the migration again:

<pre><code>def change
 add_column :profiles, :show_attribute, :boolean, default: true
end
</code></pre>

Don't forget to <code>rake db:migrate</code> and if you are using heroku <code>heroku run rake db:migrate</code>

blocks|key|697682|text|如果您不想从Rails控制台为最近的小更改创建另一个迁移文件，请执行以下操作：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|697683|ActiveRecord::Migration.change_column+:profiles,+:show_attribute,+:boolean,+:default+=>+true|code-block|syntax|javascript|697684|然后退出并重新进入rails控制台，这样DB-Changes就会生效。如果你这么做了..。|697685|Profile.new()|697686|您应该看到"show_attribute“默认值为true。|697687|对于现有记录，如果您希望保留现有的"false“设置，并且只将"nil”值更新为您的新默认值：|697688|Profile.all.each{%7Cprofile%7C+profile.update_attributes(:show_attribute+=>+(profile.show_attribute+==+nil+?+true+:+false))++}|697689|更新创建该表的迁移，这样以后的任何数据库构建都将从一开始就获得该表。还可以在数据库的任何已部署实例上运行相同的进程。|697690|如果使用"new+db迁移“方法，您可以在该迁移中更新现有的nil-value。|697691|entityMap^0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Y|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|10|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|11|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|12|8|@]|9|@]|A|$E|F]]|$1|Q|3|R|5|6|7|13|8|@]|9|@]|A|$]]|$1|S|3|T|5|6|7|14|8|@]|9|@]|A|$]]|$1|U|3|-4|5|6|7|15|8|@]|9|@]|A|$]]]|V|$]]

If you don't want to create another migration-file for a small, recent change - from Rails Console:

<pre><code>ActiveRecord::Migration.change_column :profiles, :show_attribute, :boolean, :default =&gt; true
</code></pre>

Then exit and re-enter rails console, so DB-Changes will be in-effect. Then, if you do this ...

<pre><code>Profile.new()
</code></pre>

You should see the "show_attribute" default-value as true.

For existing records, if you want to preserve existing "false" settings and only update "nil" values to your new default:

<pre><code>Profile.all.each{|profile| profile.update_attributes(:show_attribute =&gt; (profile.show_attribute == nil ? true : false)) }
</code></pre>

Update the migration that created this table, so any future builds of the DB will get it right from the onset. Also run the same process on any deployed-instances of the DB. 

If using the "new db migration" method, you can do the update of existing nil-values in that migration.

I've seen a few questions (namely <a href="https://stackoverflow.com/questions/7098602/add-a-default-value-to-a-column-through-a-migration">this one</a>) here on SO about adding a default boolean value to an existing column. So I tried the <code>change_column</code> suggestion but I mustn't be doing it right.

I tried:

<pre><code>$ change_column :profiles, :show_attribute, :boolean, :default =&gt; true
</code></pre>

Which returns <code>-bash: change_column: command not found</code>

I then ran:

<pre><code>$ rails g change_column :profiles, :show_attribute, :boolean, :default =&gt; true
</code></pre>

...and

<pre><code>$ rails change_column :profiles, :show_attribute, :boolean, :default =&gt; true
</code></pre>

Then ran <code>rake db:migrate</code>, but the value for <code>:show_attribute</code> remained <code>nil</code>. In the question I referenced above it says in PostgreSQL you need to update it manually. Since I'm using PostgreSQL I added the following in my <code>create_profiles</code> migration:

<pre><code>t.boolean :show_attribute, :default =&gt; true
</code></pre>

Can someone tell me what I'm doing wrong here?

Adding :default => true to boolean in existing Rails column

PostgreSQL

数据库

我在这里看到了一些关于向现有列添加默认布尔值的问题(即)。所以我尝试了change_column的建议，但我肯定做得不对。我试过了：$ change_column :profiles, :show_attribute, :boolean, :default => true它返回-bash: change_column:...

问将:default => true添加到现有Rails列中的布尔值
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