PMA Media Group» activerecord

You Don’t Need Custom SQL For Simple Aggregation Using ActiveRecord

Aaron Murphy — Thu, 02 Apr 2009 19:36:04 +0000

ActiveRecord allows one to execute an SQL statement directly. I prefer to avoid any ActiveRecord code that uses too much SQL syntax. This way I won’t have to rework any code if the underlying database engine was changed. Some of the simple SQL functions that one may want to do with SQL syntax might be SUM, AVG, COUNT, etc. With ActiveRecord::Calculations::ClassMethods, one can avoid specialized syntax.

#returns the total count of all people
Person.count
# returns the total count of all people with an age over 26
Person.count(:conditions => "age > 26")

The following methods are available: average, count, maximum, minimum, sum

Foreign Key Migrations Plugin

Chuck Wood — Thu, 02 Apr 2009 16:33:51 +0000

I worked on a plugin yesterday that would include all of our migration helpers. You can find it on github. Currently, it only does foreign keys work, but eventually, I’d like to have it add unique indexes on columns defined with a :unique => true attribute in the ActiveRecord migration. But for right now, here’s how to use it:

Adding a foreign key that references the id column in another table:

# The column 'id' is assumed in the referenced table.
add_foreign_key :table1, :table2_ref, :table2

Adding a foreign key that references the name column in another table:

add_foreign_key :table1, :table2_name, :table2, :name

Removing a foreign key with a custom name for the SQL constraint:

drop_foreign_key :table1, "table1_ref_to_table2"

Removing a foreign key with no custom name for the SQL constraint:

drop_foreign_key :table1, :table2_ref

ActiveRecord Destroy vs. Delete

Chuck Wood — Thu, 02 Apr 2009 16:11:22 +0000

I’ve been reading “Pro ActiveRecord” by Kevin Marshall, Chad Pytel, and Jon Yurek. They explained in the book the difference between the delete and destroy methods, which is something I had never really seen clarified anywhere. I’ve discussed it with several Rails developers and have gotten varying opinions.

Here’s what I’ve come to understand. The destroy method makes the SQL call to the database and destroys the row in the table that contains it. It does still allow you to manipulate the object in the application as long as it’s still in scope.

The delete method also makes the SQL call, but freezes the object so you cannot make changes to it. It also doesn’t make the before_destroy and after_destroy callbacks.