Migration with a decimal number with 2 trailing digits

Question

Rails 3.2
MySQL gem

I have the following in my migration:

t.decimal :pre_tax_total, default: nil, scale: 2
t.decimal :post_tax_total, default: nil, scale: 2

Based on what I read, scale:2 will produce a decimal with 2 trailing digits.

When I run the migration, and look at the table structure, I see the following:

pre_tax_total   decimal(10,0) 
post_tax_total  decimal(10,0)

Which means the values are getting truncated by the MySQL server. What is the ActiveRecord syntax to create these columns as decimal(10,2)?

Answer 1

That would be:

t.decimal :pre_tax_total, precision: 10, scale: 2

Although Rails 3 Migrations Guide skips it, the description is available in the source code:

  # Note: The precision is the total number of significant digits,
  # and the scale is the number of digits that can be stored following
  # the decimal point. For example, the number 123.45 has a precision of 5
  # and a scale of 2. A decimal with a precision of 5 and a scale of 2 can
  # range from -999.99 to 999.99.
  #
  # Please be aware of different RDBMS implementations behavior with
  # <tt>:decimal</tt> columns:
  # * The SQL standard says the default scale should be 0, <tt>:scale</tt> <=
  #   <tt>:precision</tt>, and makes no comments about the requirements of
  #   <tt>:precision</tt>.
  # * MySQL: <tt>:precision</tt> [1..63], <tt>:scale</tt> [0..30].
  #   Default is (10,0).

The last line partly explains why you got decimal(10,0).