AR 4.2 type_casting decimal without precision

[bradleypriest]

Upgrading from 4.1.8 to 4.2.0 I have run into a weird edge-case for PG decimal columns without precisions defined.
It was a bug in my application code that we didn't have the precision defined.
However, if Postgres supports it, so we probably should try to too.

gem 'activerecord', '4.2.0'
require 'active_record'

# # Print out what version we're running
puts "Active Record #{ActiveRecord::VERSION::STRING}"

# Connect to an in-memory sqlite3 database (more on this in a moment)
ActiveRecord::Base.establish_connection(
  :adapter  => 'sqlite3',
  :database => ':memory:'
)

# Create the minimal database schema necessary to reproduce the bug
ActiveRecord::Schema.define do
  create_table :users, :force => true do |t|
    t.decimal :allowance
  end
end

# Create the minimal set of models to reproduce the bug
class User < ActiveRecord::Base
end

User.new(allowance: 22.68).allowance == 22.68.to_d # fails

This seems to be due to the cast_value method in this file. https://github.com/rails/rails/blob/master/activerecord/lib/active_record/type/decimal.rb#L19

In this case because the precision.to_i is zero, the float_precision is zero and we end up calling BigDecimal.new(22.68, 0) which returns #<BigDecimal:7fd859c87cb8,'0.2267999999 9999999715 7829056959 59925652E2',45(54)>

This functionality seems to have been changed in #16333

[senny]

/cc @sgrif