Coding a migration to change a column's attribute

Rails
1

Hi All,

I want a migration to change the Amount column of the Expense table to
the precision/scale to 10/2.

It’s easy to say in one sentence. It’s a little harder to do.

1 Ran: ruby script/generate migration ChangeExpenseTbl_AmountCol
2.Got db/migrate/20100313180401_change_expense_tbl_amount_col.rb
3.Modified the migration as follows:

class ChangeExpenseTblAmountCol < ActiveRecord::Migration
def self.up
class Expense
change_column :amount, decimal :precision=>10, scale=>2
end
end

def self.down
end
end

This looks like it could be DRYed to this:

class Expense < ActiveRecord::Migration
def self.up
change_column :amount, decimal :precision=>10, scale=>2
end

def self.down
end
end

Do either of these look correct?

Thanks in Advance,
Richard

2

On 13 March 2010 19:35, RichardOnRails
[email protected] wrote:

Do either of these look correct?

Why not try it and see? Provided you have a dump of the database (and
you do have your code under source control in git (or svn) don’t you)
then if it messes something up no harm is done.

Did you check the params required for change_column from the docs (
ActiveRecord::Migration)? It
looks to me like you have a missing parameter.

Colin

3

Hi Colin,

Thanks for some additional guidance. I know I appear to be a
blithering idiot by virtue of my myriad mistakes, misunderstandings
and apparent stupidity.

Why not try it and see? Provided you have a dump of the database (and
you do have your code under source control in git (or svn) don’t you)

I didn’t try it because I have enough trouble going forward. I don’t
want to again suffer the burden of 2 days to recover from stupid
mistakes, like changing MySQL versions as I just did unnecessarily.

Dump of DB unnecessary: I only have a little toy data created by
testing.
SVN backup: I will get to that in a few weeks; right now I make
date&time-named backups of my project daily to an external hard-drive:
code and documentation.

Did you check the params required for change_column from the docs
(ActiveRecord::Migration)?
< It looks to me like you have a missing parameter.

Good site, but I didn’t find my exact requirement. Yes, I was
missing the table-name parm. Here’s my current incarnation:

class ChangeExpenseTblAmountCol < ActiveRecord::Migration
def self.up
change_column :expense, :amount, :decimal :precision =>
10, :scale => 2
end

def self.down
end
end

No matter what permutation I try, the db:migrate task finds an error
(usually syntax).

When I was in USAF Radio School at the beginning of the Korean “Police
Action”, I discovered the Cryptographic School next door and “knew” I
belonged there. But the effort to crack the code proves I was wrong
those many years ago, 'cause I can’t crack this code.

Does anything leap out at you?

As usual, thanks for the gift of your time and expertise,
Richard

4

OK, Collin,

Thanks to that migrations link you posted last time, I saw another
format, the old C-style coding as opposed to the new Rails-DSL-
style. Below is the migration code, the db:migration script
execution and the results in the database. You don’t need to look at
all that stuff. I just added to show that the old style does work and
pique your curiosity that the new styles don’t seem to.

Again, Colin, thanks for all your help on this and other programming
“hardships”.

== execution results ===

• This format works:
class ChangeExpenseTblAmountCol < ActiveRecord::Migration
def self.up
change_column( “expenses”, “amount”, “decimal(10, 2)” )
end

def self.down
end
end

• Apply the migration
K:_Projects\Ruby_Rails_Apps_EIMS\RTS>rake db:migrate
(in K:/_Projects/Ruby/_Rails_Apps/_EIMS/RTS)
== ChangeExpenseTblAmountCol: migrating

– change_column(“expenses”, “amount”, “decimal(10, 2)”)
-> 0.7030s
== ChangeExpenseTblAmountCol: migrated (0.7190s)

K:_Projects\Ruby_Rails_Apps_EIMS\RTS>

• Testing the database confirms it:
mysql> describe expenses;
±-----------±--------------
| Field | Type
±-----------±--------------
| id | int(11)
| purpose | varchar(255)
| vendor | varchar(255)
| type | varchar(255)
| date | date
| amount | decimal(10,2) [snip]

5

On Mar 13, 11:13 pm, RichardOnRails
[email protected] wrote:

class ChangeExpenseTblAmountCol < ActiveRecord::Migration
def self.up
change_column :expense, :amount, :decimal :precision =>
10, :scale => 2
end

The first parameter is the table name ( ie plural) and you’re missing
a comma towards the end.

Fred

6

On Sat, 2010-03-13 at 15:45 -0800, Frederick C. wrote:

The first parameter is the table name ( ie plural) and you’re missing
a comma towards the end.

in the thought that give a man a fish and feed him for a day and teach a
man to fish, he can feed himself the rest of his life…

rake SOME_COMMAND --trace

gives you more meaningful output on what went wrong

Craig

–
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believed to be clean.

7

Hi Frederick,

Thanks for taking the time to look at my newbie code. I now have both
styles:
change_column :expenses, :amount, :decimal, :precision =>
10, :scale => 2 # DSL-style
change_column( “expenses”, “amount”, “decimal(10,
2)” ) # C-style
working. Thank you for pointing the things I was missing when I
fumbled here.

I wont have any more problems in this area. But I’m moving on to a
new area now :slight_smile:

Best wishes,
Richard

On Mar 13, 7:45 pm, Frederick C. [email protected]

8

Hi Craig,

As I mentioned to Frederick, I’ve got both the DSL-style and C-Style
for change_column working after a bunch of stumbles. I expect to be
much better with future migrations I code. In that context, I’m sure
the “–trace” option you pointed out will be very helpful. Thanks for
taking the time to look in on this problem.

Best wishes,
Richard