blocks|key|4637231|text|如果在升级到rails+4.2.4+(也适用于rails+4.1.5)时出现此错误，请尝试使用此版本的mysql2：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4637232|gem+'mysql2',+'~>+0.3.18'|code-block|syntax|javascript|4637233|显然，mysql2仍然与更新版本的rails不兼容，因为在我2015年9月8日回答这个问题时，rails+4.2.4是相当新的，所以在你的Gem文件中使用上一行并运行：|4637234|bundle+install+|4637235|你应该可以走了|4637236|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

If you have this error when upgrading to rails 4.2.4 (also with rails 4.1.5) try using this version of mysql2:

<pre><code>gem 'mysql2', '~&gt; 0.3.18'
</code></pre>

Apparently mysql2 isn't still compatible with newer version of rails because rails 4.2.4 is pretty new as the time of answering this question by me 8 September 2015 so use the above line in your Gem file and run: 

<pre><code>bundle install 
</code></pre>

You should be good to go

blocks|key|4637285|text|当我在最新版本(0.4.0)之前指定一个mysql2+gem版本时，它对我来说是有效的。由于某些原因，Rails+4.2.4和gem+0.4.0存在问题。所以，为了解决这个问题，我只指定了之前发布的gem+:+0.3.20，它对我来说工作得很好！|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4637286|gem+'mysql2',+'~>+0.3.20'
bundle+install|code-block|syntax|javascript|4637287|你可以在这里查看所有的gem版本：https://rubygems.org/gems/mysql2/versions|offset|length|4637288|entityMap|0|LINK|mutability|MUTABLE|url|https://rubygems.org/gems/mysql2/versions^0|0|0|H|15|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]]]]

It worked for me when I specified a mysql2 gem version before the newest one (0.4.0). 
For some reason there is a problem with Rails 4.2.4 and that gem 0.4.0. So, to solve the problem I just specified the previous gem released: 0.3.20 and it worked fine for me!

<pre><code>gem 'mysql2', '~&gt; 0.3.20'
bundle install
</code></pre>

You can check all the gems versions here: <a href="https://rubygems.org/gems/mysql2/versions">https://rubygems.org/gems/mysql2/versions</a>

blocks|key|131139|text|更改为|type|unstyled|depth|inlineStyleRanges|entityRanges|data|131140|gem+'mysql2',+'~>+0.3.18'|code-block|syntax|javascript|131141|在你的Gemfile里。|131142|官方mysql2+Github上的这条帖子说要这样做。如果您是rails版本4.x.x，则需要声明该版本号。|131143|https://github.com/brianmario/mysql2/issues/675|offset|length|131144|然后运行bundle+update+mysql2。|style|CODE|131145|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|0|0|0|1B|0|0|4|K|0^^$0|@$1|2|3|4|5|6|7|Z|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|10|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|11|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|12|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|13|8|@]|9|@$M|14|N|15|1|16]]|A|$]]|$1|O|3|P|5|6|7|17|8|@$M|18|N|19|Q|R]]|9|@]|A|$]]|$1|S|3|-4|5|6|7|1A|8|@]|9|@]|A|$]]]|T|$U|$5|V|W|X|A|$Y|L]]]]

Change to

<pre><code>gem 'mysql2', '~&gt; 0.3.18'
</code></pre>

in your Gemfile.

This thread on the official mysql2 Github says to do this. You need to declare that version number if you're rails version 4.x.x.

<a href="https://github.com/brianmario/mysql2/issues/675" rel="nofollow noreferrer">https://github.com/brianmario/mysql2/issues/675</a>

Then run <code>bundle update mysql2</code>.

blocks|key|4636973|text|在升级到Rails4.1之后，我得到了同样的错误，我设法通过更新mysql2解决了这个问题。在您的rails应用程序文件夹中运行以下命令：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4636974|$+bundle+update+mysql2|code-block|syntax|javascript|4636975|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 got the same error after an upgrade to Rails 4.1 and I managed to resolve it by updating mysql2. Run this in your rails app folder:

<pre><code>$ bundle update mysql2
</code></pre>

blocks|key|131068|text|如果您使用的是更新版本的rails+>4，则可能会出现此问题|type|unstyled|depth|inlineStyleRanges|entityRanges|data|131069|做这两个简单的步骤，它将会起作用。打开您的Gemfile并找到以下行|131070|gem+'mysql2'|code-block|syntax|javascript|131071|将该行替换为特定的mysql版本，如下所示|131072|gem+'mysql2',+'~>+0.3.18'|131073|现在停止服务器并运行bundle|131074|bundle+install|131075|现在重新启动您的服务器。应该能行得通。|131076|rails+s|131077|entityMap^0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|X|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Y|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|Z|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|10|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|11|8|@]|9|@]|A|$]]|$1|O|3|P|5|F|7|12|8|@]|9|@]|A|$G|H]]|$1|Q|3|R|5|6|7|13|8|@]|9|@]|A|$]]|$1|S|3|T|5|F|7|14|8|@]|9|@]|A|$G|H]]|$1|U|3|-4|5|6|7|15|8|@]|9|@]|A|$]]]|V|$]]

This issue may occur if you're using newer version of rails > 4

Do these two simple steps, it will work.
Open your Gemfile and find the below line

<pre><code>gem 'mysql2'
</code></pre>

replace that line with a specific mysql version like below

<pre><code>gem 'mysql2', '~&gt; 0.3.18'
</code></pre>

Now stop the server and run bundle

<pre><code>bundle install
</code></pre>

Now restart your server. It should work.

<pre><code>rails s
</code></pre>

blocks|key|131011|text|它不会加载mysql2+gem，因为新版本的mysql2(0.4.1)+gem无法加载mysql2_adaptor。这对我很有效。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|131012|gem+'mysql2',+'~>+0.3.13'|code-block|syntax|javascript|131013|然后运行|131014|bundle+install|131015|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|$]]

It doesn't load mysql2 gem because new version of mysql2(0.4.1) gem unable to load the mysql2_adaptor. This is working for me. 

<pre><code>gem 'mysql2', '~&gt; 0.3.13'
</code></pre>

and run 

<pre><code>bundle install
</code></pre>

blocks|key|4637015|text|我遇到了同样的错误，这是因为Rails4.1需要最低版本为0.3.13的mysql2，最高兼容版本为0.3.11。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4637016|因此，我编辑了文件c:\RailsInstaller\Ruby1.9.3\lib\ruby\gems\1.9.1\gems\activerecord-4.1.1\lib\active_record\connection_adapters\mysql2_adapter.rb，并将行gem+'mysql2',+'~>+0.3.13'更改为gem+'mysql2',+'~>+0.3.11'，到目前为止它仍然有效。|offset|length|style|CODE|4637017|entityMap^0|0|9|3J|3W|P|4O|P|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@$D|L|E|M|F|G]|$D|N|E|O|F|G]|$D|P|E|Q|F|G]]|9|@]|A|$]]|$1|H|3|-4|5|6|7|R|8|@]|9|@]|A|$]]]|I|$]]

I had the same error and this is because Rails 4.1 requires minimum mysql2 version 0.3.13, and maximum compatible with Windows is version 0.3.11.

So I edited file <code>c:\RailsInstaller\Ruby1.9.3\lib\ruby\gems\1.9.1\gems\activerecord-4.1.1\lib\active_record\connection_adapters\mysql2_adapter.rb</code> and changed line <code>gem 'mysql2', '~&gt; 0.3.13'</code> to <code>gem 'mysql2', '~&gt; 0.3.11'</code>, and it works so far.

blocks|key|4637185|text|下面是我如何解决这个问题的：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4637186|+bundle+config
+bundle+config+--delete+without
+bundle+install+--deployment+--without+development+test+postgres|code-block|syntax|javascript|4637187|致词：How+do+you+undo+bundle+install+--without|offset|length|4637188|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/9765007/how-do-you-undo-bundle-install-without/15145462#15145462^0|0|0|3|14|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]]]]

Here is how I fixed this:

<pre><code> bundle config
 bundle config --delete without
 bundle install --deployment --without development test postgres
</code></pre>

Credits:
<a href="https://stackoverflow.com/questions/9765007/how-do-you-undo-bundle-install-without/15145462#15145462">How do you undo bundle install --without</a>

blocks|key|4637380|text|它不加载mysql2+gem，因为新版本的mysql2+(>=+0.4.0)+gem无法加载mysql2_adaptor。你能试试这个吗？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4637381|gem+'mysql2',+'~>+0.3.13'|offset|length|style|CODE|4637382|希望它能起作用。|4637383|entityMap^0|0|0|P|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|M|8|@$D|N|E|O|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|P|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|K|$]]

It doesn't load mysql2 gem because new version of mysql2 (>= 0.4.0) gem unable to load the mysql2_adaptor. Can you try this?

<code>gem 'mysql2', '~&gt; 0.3.13'</code>

Hopefully, it should work.

blocks|key|130822|text|我解决了这个问题，安装了gem+gem+local+(gem+install+mysql2，bundle+install)，并将以下行添加到mysql2：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|130823|gem+'mysql2'|offset|length|style|CODE|130824|在database.yml中设置mysql2适配器|130825|adapter:+mysql2|130826|也很重要！|130827|entityMap^0|0|0|C|0|0|0|F|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Q|8|@$D|R|E|S|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|T|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|U|8|@$D|V|E|W|F|G]]|9|@]|A|$]]|$1|L|3|M|5|6|7|X|8|@]|9|@]|A|$]]|$1|N|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|O|$]]

I solved the problem, installing the mysql2 gem local (gem install mysql2, bundle install) and adding the following line to the Gemfile:

<code>gem 'mysql2'</code>

Setting the mysql2 adapter in database.yml 

<code>adapter: mysql2</code>

was also important!

blocks|key|131104|text|我是Ruby+on+Rails和网站的新手，但听到的是对我有用的东西。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|131105|我不得不更改我的gem文件，gem+'+mysql2‘为gem+'mysql2'，'~>+0.3.13’，然后在rails中我输入包安装，然后我尝试rails+s，得到错误，然后我尝试包更新mysql2，然后在rails中输入rails+s，它工作了|131106|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|$]]

I'm brand spanking new to Ruby on Rails and websites but hears what worked for me.

I had to change my gemfile, gem 'mysql2' to gem 'mysql2', '~> 0.3.13'
then in rails i typed bundle install
then i tried rails s and got errors
so then i tried bundle update mysql2
then in rails typed rails s, and it worked

blocks|key|4637516|text|我通过安装libmysqlclient-dev解决了这个问题。sudo+aptitude安装libmysqlclient-dev，然后运行bundle。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4637517|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 solved the problem, installing the libmysqlclient-dev.
sudo aptitude install libmysqlclient-dev
and later run bundle.

blocks|key|4637494|text|我之前安装了mysql2+0.4.5，但出现了这个错误，所以我通过以下命令安装了另一个版本的mysql2：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4637495|gem安装mysql2+--版本0.3.20|4637496|希望这能解决你的问题。|4637497|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

I have previously installed mysql2 0.4.5 but that was giving me this error so i have installed another version of mysql2 by:

gem install mysql2 --version 0.3.20

Hope this solves your problem.

<pre><code>Gem::LoadError
Specified 'mysql2' for database adapter, but the gem is not loaded.
Add `gem 'mysql2'` to your Gemfile
</code></pre>

This error occurred while loading the following files:

<pre><code>active_record/base
</code></pre>

This is the error I get on running rails server.

<pre><code>The mysql2 gem has been added to the Gemfile as well.
</code></pre>

I've done <code>bundle install</code>, and tried restarting the server but still get the error.

Gem::LoadError for mysql2 gem, but it's already in Gemfile

服务器

Gem::LoadErrorSpecified 'mysql2' for database adapter, but the gem is not loaded.Add `gem 'mysql2'` to your Gemfile加载以下文件时发生此错误：active_record/base这是我在运行rails服务器时遇到的错误。The mysql2 gem has been added to 

问mysql2 gem的Gem::LoadError，但它已经在Gemfile中
EN

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问mysql2 gem的Gem::LoadError，但它已经在Gemfile中
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