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).