Question

Getting started with Rails and I’m stuck with a block operation. I want to loop through a model and update integer values for a new field by calculating difference between two datetime fields. Doing this in the console:

MyModel.all.each do |m|
  m.new_integer_field = m.existing_datetime_field - m.parent_object.existing_datetime_field
  m.save!
end

The result is

 NoMethodError: undefined method `-' for nil:NilClass

It works on one record if I do:

m = MyModel.find(1)
m.new_integer_field = m.existing_datetime_field - m.parent_object.existing_datetime_field
m.save

I guess it’s a basic syntax thing, but couldn’t find obvious explanations. Grateful to get some pointers forward.

Answer 1

The m.existing_datetime_field in one of the instances was nil and therefore the operation didn’t work. You need to check for nil values if not sure:

MyModel.all.each do |m|
  unless m.existing_datetime_field == nil
    m.new_integer_field = m.existing_datetime_field - m.parent_object.existing_datetime_field
  m.save!
end