Question

consider that i have a migration as follows

create_table :dummies do |t|
  t.decimal :the_dummy_number
end 

i instantiate like the following

dummy = Dummy.new
dummy.the_dummy_number = "a string"
puts dummy.the_dummy_number

the output for the above is

 0.0

how did this happen? since i assign a wrong value shouldn’t it raise an error?

The biggest problem is the following.

Since it automatically converts my validate method fails miserably.

update-the validate method

 validate :is_dummy_number_valid, :the_dummy_number
 def is_dummy_number_valid
    read_attribute(:the_dummy_number).strip()
 end

Answer 1

The reason that this does not work as you expect is that the underlying ruby implementation of BigDecimal does not error when passed a string.

Consider the following code

[ 'This is a string', '2is a string', '2.3 is also a string', 
  '   -3.3 is also a string'].each { |d| puts "#{d} = #{BigDecimal.new(d)}" }

This is a string = 0.0
2is a string = 2.0
2.3 is also a string = 2.3
   -3.3 is also a string = -3.3

So BigDecimal scans the string and assigns anything at the beginning of the string that could be a decimal to its value.

If you set your model up like this

class Dummy < ActiveRecord::Base

   validates_numericality_of :the_dummy_number

end

Then the validation should work fine

>> d=Dummy.new(:the_dummy_number => 'This is a string')
=> #<Dummy id: nil, the_dummy_number: #<BigDecimal:5b9230,'0.0',4(4)>, created_at: nil, updated_at: nil>

>> puts d.the_dummy_number
0.0
=> nil
>> d.valid?
=> false

>> d.errors
=> #<ActiveRecord::Errors:0x5af6b8 @errors=#<OrderedHash
  {"the_dummy_number"=>[#<ActiveRecord::Error:0x5ae114 
   @message=:not_a_number, @options={:value=>"This is a string"}

This works because the validates_numericality_of macro uses the raw_value method to get at the value before it was typecast and assigned to the internal decimal value.