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Database Migrations

DB Creation and Setup

To first setup the db, do

rake db:create
rake db:automigrate

Altering later

Afterwards, you can add columns or change certain attributes of columns simply by making the necessary changes in the model and then running

rake db:autoupgrade

to do a non-destructive upgrade or

rake db:automigrate

to do a destructive upgrade that will clear the database.

BEWARE

“Non-destructive” does not necessarily mean that something bad can’t happen. It’s always good policy to backup the database before messing with it if you care about the data.

Default Values

If you are adding a column, you can set the :default attribute in the model, an it will set the value of the new attribute to that default value for each existing row.

For example, if you have a Person class with just an id attribute and 2 people in the db, and then you add

property :name, String, :default => "John Smith"

to the model file at app/models/person.rb and then run

rake db:autoupgrade

then if you enter the console and inspect the people that had already been created, you will see:

 $ merb -i
 # annoying output you don't care about
 irb(main):001:0> Person.all
 ~ SELECT "id", "name" FROM "people" ORDER BY "id"
 => [#<Person id=1 name="John Smith">, #<Person id=2 name="John Smith">]

The People had their names automatically set to “John Smith” because that was the default value.

DM-Migrations

You can use the dm-migrations plugin to make migrations to help with more advanced tasks.

There’s some info (just a link as of now) here.

TODO explain using dm-migrations

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