Question

I could always risk botching my dev database (PostgreSQL) & rebuild it but thought I’d ask first to see if someone had experience.

Ideally all data would be truncated to its integer amount: (eg: 5.7 would become 5) or perhaps it would round values (5.7 becomes 6)? or perhaps it simply nullifies or zeros out all values and data is lost? I suppose the worst outcome would be unreliable data (5.7 becomes 23).

Is there a general rule of thumb for these types of migrations?

class ChangeBookFromDecimalToInteger < ActiveRecord::Migration
  def self.up
   change_column :book, :price, :integer
  end

  def self.down
   change_column :book, :price, :decimal
  end
end

Answer 1

ActiveRecord will send an ALTER TABLE ALTER COLUMN … TYPE to the database and the database will perform the type conversion. PostgreSQL will convert decimal to int using round:

=> create table with_decimal (n decimal(11, 6));
=> insert into with_decimal (n) values (1.0),(1.1),(1.5),(1.6),(1.9);
=> insert into with_decimal (n) values (-1.0),(-1.1),(-1.5),(-1.6),(-1.9);
=> select * from with_decimal;
     n     
-----------
  1.000000
  1.100000
  1.500000
  1.600000
  1.900000
 -1.000000
 -1.100000
 -1.500000
 -1.600000
 -1.900000
(10 rows)

=> alter table with_decimal alter column n type int;
=> select * from with_decimal;
 n  
----
  1
  1
  2
  2
  2
 -1
 -1
 -2
 -2
 -2
(10 rows)

Note that round(numeric) rounds to the nearest integer.

If you want specific conversion behavior, you should say so with a USING in the ALTER TABLE:

The optional USING clause specifies how to compute the new column value from the old; if omitted, the default conversion is the same as an assignment cast from old data type to new. A USING clause must be provided if there is no implicit or assignment cast from old to new type.

If you need a USING clause, you have to issue the ALTER TABLE by hand as ActiveRecord doesn’t know anything about USING, for example:

def up
    connection.execute(%q{
        alter table books
        alter column price 
        type integer
        using trunc(price * 100)
    })
end