Even though Rails shields us from having to resort to SQL, it’s still good to know how to do things directly. One thing I wanted to do today was find all the records that contain “blahblahblah” and replace it with “blah_dee_blah_dee_blah”. Here’s how I did it:

update wp_3_posts
set POST_CONTENT = replace(POST_CONTENT,
'http://www.volcanicmarketing.com/wp-content/',
'http://volcanic.sitespress.net/wp-content/uploads/');

As you may or may not have guessed, this wasn’t for a Rails app. It was for my WordPress system. I had moved from single wordpress sites to a server that runs WordPress Mu. One of the changes that resulted from this was that images contained inside my posts didn’t automatically adjust. The above code fixes this. Let’s deconstruct it by looking at by looking at the structure of the command:

update TABLE_NAME
set FIELD_NAME = replace(FIELD_NAME,
'find this string',
'replace found string with this string');

I got the info from http://www.mydigitallife.info/2007/04/23/how-to-find-and-replace-text-in-mysql-database-using-sql/

Q: Is there a way to make a migration from an existing database?

A. Yes, but you need to be more specific with this question because there are two ways.

The first approach will dump the schema of your existing database into a migration.

The second will dump the data as well. Why would you want to dump the data? If you’re moving machines or changing from a MySQL to a POSTGREs db, this might be useful.

Approach 1: Dump schema

Hint: Type in “rake -T” into your terminal. The will show you a list of all the rake tasks available to you. Everything you need is here.

Step 1: rake db:schema:dump   #this will dump the schema into a file called schema.rb

Step 2: rake db:schema:to_migration #this will turn your schema.rb file into a migration file

Approach 2: Dump schema + data

You’re going to need the YAMLdb plugin by Adam Wiggins at Heroku. To grab it from: http://github.com/adamwiggins/yaml_db/

Follow the instructions on the github page.

Conversation from #rubyonrails IRC channel

pipegeek: Is there a straightforward way, given a list of ids, fetch all corresponding records, ignoring invalid ids?

pipegeek: ie, something like Model.find([1,2,3,4,5]), but without the requirement that all the ids be matched

pipegeek: err, should clarify (if it isn’t already obvious) I’m speaking about activerecord

therrg: pipegeek: Model.find_all_by_id([1,2,3,4,5]) does what you want

pipegeek: oh!  Damn, I feel foolish for not having checked that—figured the find_all_by_ methods behaved the same way

pipegeek: thanks, therrg

therrg: yw

melvinram: btw, what is your usage scenario?

melvinram: trying to see when that would be useful

pipegeek: Well, I have a list of ids generated much earlier by some other tool that added a bunch of records

pipegeek: I need to do something to all of those records.  Only, it’s possible that some have been DELETEd in the meantime

melvinram: ahe i see