Skip to main content

The Default Debacle

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.

Today I diverged from the examples in the book again and encountered an issue with the word 'default'. It turns out that default is a SQL keyword depending on the context of the expression it is used in. I wanted to store a default value for a record in a table. This is illustrated in the table below:


The table tracks the 'default' or usual state of each user and their current 'actual' state. This seemed simple enough. The add_column call in my migration file had no problem with the column name 'default'. However, when I tried to add items to the table, the column name 'default' caused an SQL error. This is because the word 'default' can have a special meaning in an insert query.

There may be a way to escape 'default' so that it's accepted as a column name, but I don't know what it is. (If you do, please comment! Thanks!) So, the simple path seemed to be to just rename the column. But... when I used rename_column, I also received SQL errors. The solution that finally worked was to first use remove_column to get rid of the column all together. Fortunately since insert didn't like 'default' there was no data in the column to worry about deleting. I then added the column default_value using the add_column call.

Just to be on the safe side, I did not implement the opposite of add_column in the self.down method of the migration file.


Popular posts from this blog

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

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 rule can be used to convert a differential operator in terms of one variable into a series of differential operators in terms of othe…

Lost Phone

We were incredibly lucky to have both been in university settings when our kids were born.  When No. 1 arrived, we were both still grad students.  Not long after No. 2 arrived, (about 10 days to be exact), mom-person defended her dissertation and gained the appellation prependage Dr. 

While there are lots of perks attendant to grad school, not the least of them phenomenal health insurance, that’s not the one that’s come to mind for me just now.  The one I’m most grateful for at the moment with respect to our kids was the opportunities for sheer independence.  Most days, we’d meet for lunch on the quad of whatever university we were hanging out at at the time, (physics research requires a bit of travel), to eat lunch.  During those lunches, the kids could crawl, toddle, or jog off into the distance.  There were no roads, and therefore no cars.  And, I realize now with a certain wistful bliss I had no knowledge of at the time, there were also very few people at hand that new what a baby…

Lab Book 2014_07_10 More NaI Characterization

Summary: Much more plunking around with the NaI detector and sources today.  A Pb shield was built to eliminate cosmic ray muons as well as potassium 40 radiation from the concreted building.  The spectra are much cleaner, but still don't have the count rates or distinctive peaks that are expected.
New to the experiment?  Scroll to the bottom to see background and get caught up.
Lab Book Threshold for the QVT is currently set at -1.49 volts.  Remember to divide this by 100 to get the actual threshold voltage. A new spectrum recording the lines of all three sources, Cs 137, Co 60, and Sr 90, was started at approximately 10:55. Took data for about an hour.
Started the Cs 137 only spectrum at about 11:55 AM

Here’s the no-source background from yesterday
In comparison, here’s the 3 source spectrum from this morning.

The three source spectrum shows peak structure not exhibited by the background alone. I forgot to take scope pictures of the Cs137 run. I do however, have the printout, and…