# Multiple databases - "stuck" after unknown database

**URL:** <https://www.ruby-forum.com/t/multiple-databases-stuck-after-unknown-database/153373>\
**Category:** Rails\
**Created:** [November 15, 2008, 10:39pm UTC](https://www.ruby-forum.com/t/multiple-databases-stuck-after-unknown-database/153373 "2008-11-15T22:39:57Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![rich](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/35a633/32.png) [@rich](https://www.ruby-forum.com/u/rich)\
**Post date:** [November 15, 2008, 10:39pm UTC](https://www.ruby-forum.com/t/multiple-databases-stuck-after-unknown-database/153373/1 "2008-11-15T22:39:57Z")

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I’m implementing a one database per subdomain app, which is pretty  
simple using the before\_filter described on the wiki:  
[http://wiki.rubyonrails.org/rails/pages/HowtoUseMultipleDatabases](http://wiki.rubyonrails.org/rails/pages/HowtoUseMultipleDatabases).

I just do this in my hijack\_db method:

return unless request.subdomains.first  
db\_name = “#{request.subdomains.first}\_#{RAILS\_ENV}”  
ActiveRecord::Base.establish\_connection  
(ActiveRecord::Base.configurations[RAILS\_ENV].merge({ ‘database’ =\>  
db\_name }))

My problem happens when someone hits a subdomain that doesn’t exist.  
It tries to connect to a database that doesn’t exist and throws the  
Mysql::Error 'Unknown database …" Then on the next request, before  
it ever makes it to the ApplicationController, when it’s making a  
connection to the database for the query cache, it tries to use the  
last connection specification (the invalid one) and I just get the  
unknown database error again before my hijack\_db ever gets a chance to  
connect to a good db. At that point, the app is “stuck” trying to  
connect to the unknown db.

I have a fix for this but I’m wondering if it’s a hack or if it’s  
something I should actually try submitting as a patch to Rails (I’ve  
never contributed before).

My fix/hack is that I just wrap the connection method in  
ConnectionPool in a begin/rescue and catch the unknown database error  
and “revert” the database in the connection specification to the one  
from database.yml so that (assuming the database.yml has a valid db)  
the next request will succeed.

Anyone have any opinions on this?

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<div class="post-metadata">

**Author:** ![rich](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/35a633/32.png) [@rich](https://www.ruby-forum.com/u/rich)\
**Post date:** [November 15, 2008, 11:36pm UTC](https://www.ruby-forum.com/t/multiple-databases-stuck-after-unknown-database/153373/2 "2008-11-15T23:36:09Z")

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Hi Rich,

Rich wrote:

> It tries to connect to a database that doesn’t exist and throws the  
> Mysql::Error 'Unknown database …"
> 
> I have a fix for this but I’m wondering if it’s a hack

‘Hack’ can have such a negative connotation. I like ‘monkey patch’  
better  
😉

> My fix/hack is that I just wrap the connection method in  
> ConnectionPool in a begin/rescue and catch the unknown  
> database error and “revert” the database in the connection  
> specification to the one from database.yml so that (assuming  
> the database.yml has a valid db) the next request will succeed.

I’m wondering why the same result couldn’t be had by wrapping the  
connection  
in your hijack\_db method. I don’t know enough about the nitty-gritty of  
connections, so plz take this as a question that, if you’ve got time,  
I’m  
interested in. I’ve got a project that might want this same sort of db  
setup (one per subdomain).

Thanks,  
Bill

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<div class="post-metadata">

**Author:** ![rich](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/35a633/32.png) [@rich](https://www.ruby-forum.com/u/rich)\
**Post date:** [November 15, 2008, 11:44pm UTC](https://www.ruby-forum.com/t/multiple-databases-stuck-after-unknown-database/153373/3 "2008-11-15T23:44:21Z")

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Did you, by any chance, play in the NBA in the 70s and 80s? Sorry you  
probably get that a lot.

The connection in hijack\_db never gets called if it’s an unknown  
database. I’m no expert either, but it appears that at some point  
before ApplicationController starts handling the request, Rails is  
doing something with the query cache and it needs to establish a  
connection for that. That’s where the unknown db error is raised so I  
need to catch it there, lower down.