Modifying Migrations

As I continue my study of RubyOnRails using Build Your Own Ruby On Rails Web Applications by Patrick Lenz, I'm including expansions on certain topics that piqued my interest. First, here are a few useful links if you're reading the book too.

book web site

book errata

ruby on rails docs

I learned a little bit more about how rake handles automatic migrations today. I suspected that like it's similarly named counterpart rake might determine which migrations to apply based on the last change date of the migration file. I was wrong.

I had created a new migration to add a column to one of my models. I ran rake and everything went without a hitch. Then, about two minutes later, I realized that I still needed to add one more column to that particular model. I didn't want to create another migration file for the second new column. "No problem," I reasoned. "I'll just add a new column to my latest migration file." I did this, ran rake, and nothing! As it turns out, rake doesn't care a bit about the time stamp on the migration file. What rake does keep track of is the last migration applied. It does this by looking at a table that rails inserts into your database. The name of the table is schema_info. The (only?) record in the table stores the number of the latest migration that was applied.

So, I did what I probably should have done in the first place. I used:

rake db:migrate VERSION 7

to move back to the previous migration. When I did this, the affects of applying the current migration, (adding a new column), were undone. Then, I modified my migration file to add the second new column and ran

rake db:migrate

rake now saw that there was a migration ahead of the one being used and re-applied my new migration file.

Please add your questions and comments/corrections to the comments below! Thanks!

Comments

More Cowbell! Record Production using Google Forms and Charts

First, the what : This article shows how to embed a new Google Form into any web page. To demonstrate ths, a chart and form that allow blog readers to control the recording levels of each instrument in Blue Oyster Cult's "(Don't Fear) The Reaper" is used. HTML code from the Google version of the form included on this page is shown and the parts that need to be modified are highlighted. Next, the why : Google recently released an e-mail form feature that allows users of Google Documents to create an e-mail a form that automatically places each user's input into an associated spreadsheet. As it turns out, with a little bit of work, the forms that are created by Google Docs can be embedded into any web page. Now, The Goods: Click on the instrument you want turned up, click the submit button and then refresh the page. Through the magic of Google Forms as soon as you click on submit and refresh this web page, the data chart will update immediately. Turn up the:

Cool Math Tricks: Deriving the Divergence, (Del or Nabla) into New (Cylindrical) Coordinate Systems

Now available as a Kindle ebook for 99 cents ! Get a spiffy ebook, and fund more physics The following is a pretty lengthy procedure, but converting the divergence, (nabla, del) operator between coordinate systems comes up pretty often. While there are tables for converting between common coordinate systems , there seem to be fewer explanations of the procedure for deriving the conversion, so here goes! What do we actually want? To convert the Cartesian nabla to the nabla for another coordinate system, say… cylindrical coordinates. What we’ll need: 1. The Cartesian Nabla: 2. A set of equations relating the Cartesian coordinates to cylindrical coordinates: 3. A set of equations relating the Cartesian basis vectors to the basis vectors of the new coordinate system: How to do it: Use the chain rule for differentiation to convert the derivatives with respect to the Cartesian variables to derivatives with respect to the cylindrical variables. The chain

The Valentine's Day Magnetic Monopole

There's an assymetry to the form of the two Maxwell's equations shown in picture 1. While the divergence of the electric field is proportional to the electric charge density at a given point, the divergence of the magnetic field is equal to zero. This is typically explained in the following way. While we know that electrons, the fundamental electric charge carriers exist, evidence seems to indicate that magnetic monopoles, the particles that would carry magnetic 'charge', either don't exist, or, the energies required to create them are so high that they are exceedingly rare. That doesn't stop us from looking for them though! Keeping with the theme of Fairbank[1] and his academic progeny over the semester break, today's post is about the discovery of a magnetic monopole candidate event by one of the Fairbank's graduate students, Blas Cabrera[2]. Cabrera was utilizing a loop type of magnetic monopole detector. Its operation is in concept very sim

Copasetic Flow

Search This Blog