Rails check if field value is the default value for that field

Question

I have a field named number on the Address model, which has a default of 0 set at database level. If I do Address.new.number, I will get 0 back.

Now, I want to trigger a before_create callback only if number != <default value>. So, I will have before_create :callback, unless: <condition>. How can I check this using some Rails "logic"? I know I can compare to 0, but I'd rather compare to a dynamical value, as retrieved by Rails.

Answer 1

Default values are accessible in Address.column_defaults as a hash of {"column" => default}:

Address.column_defaults
#=> {"number" => 0, "street" => nil, ...}

You can also use the <attribute>_changed? methods Rails provides. This will be false unless you initialize the object with a non-default value.

Address.new.number_changed? #=> false
Address.new(number: 1).number_changed? #=> true

So your before_create can look like:

before_create :callback, if: :number_changed?