uninitialized constant MysqlCompat::MysqlRes error when starting Rails

After a few upgrades, re-installs and moving to new servers I have been getting this error message uninitialized constant MysqlCompat::MysqlRes when I start rails. (Passenger or Thin)

I have been using mysqlplus gem but wanted to move back to MySQL 2.8.1 gem and I think that is when this started to popup more often. (after looking closer I have servers with ruby 1.8.7 and 1.8.6 using the mysql 2.8.1 gem without issues… )

After firing up thin using strace on my CentOS server I found that for some reason the mysql gem is looking for libmysqlclient.so.16 in /lib64 /usr/lib64 … and not in /opt/mysql/lib where these libraries are located. What I did to fix this issue was to create a link in /usr/lib64/ pointing to the libmysqlclient.so.16 library and voila! the problem is fixed.

PS I think you can set the mysql-lib-dir when you install the mysql gem but I have not tested this yet. I’m thinking something like this might have fixed the issue too(not tested):

[root@server]# gem install mysql -- --with-mysql-dir=/opt/mysql \
                                    --with-mysql-lib=/opt/mysql/lib \
                                    --with-mysql-include=/opt/mysql/include

 

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