"NameError: uninitialized constant" when fake exists in same file - ruby-on-rails

Question

Say I have:
app/models/food/fruit.rb:
module Food
class Fruit
class_attribute :field_mapping
self.field_mapping = MyOrg::Trees::Details.field_mapping
# other things including:
end
end
In local/development mode, I can open a console and do/get this:
rails console
[1] pry(main)> Food::Fruit
NameError: uninitialized constant MyOrg::Trees::Details
MyOrg::Trees::Details will eventually be defined via a gem, but until then I want to fake it by adding the following to the top of the same file; why isn't this doing the trick?
module MyOrg
module Trees
class Details
def field_mapping
end
end
end
end
Note: when I paste the above into a local/development console, I can then successfully do this:
[2] pry(main)> MyOrg::Trees::Details.new.field_mapping
=> nil

Answer 1

When I added the above fake, I accidentally started trying to reference it to test what I was doing...
rails console
[1] pry(main)> MyOrg::Trees::Details
NameError: uninitialized constant MyOrg::Trees::Details
...but I should have called Food::Fruit as before, and I would have isolated the issue...
rails console
[1] pry(main)> Food::Fruit
NoMethodError: undefined method `field_mapping' for MyOrg::Trees::Details:Class
...ah, I need to make it a class method...
module MyOrg
module Trees
class Details
def self.field_mapping
end
end
end
end
...and then...
rails console
[1] pry(main)> Food::Fruit
NoMethodError: undefined method `values' for nil:NilClass
So, I appeased the undefined method error, and now have a new problem, which is that further down the original code there is actually another reference to field_mapping (field_mapping.values) which now has to be appeased...
module MyOrg
module Trees
class Details
def self.field_mapping
{}
end
end
end
end
...and now...
rails console
[1] pry(main)> Food::Fruit
=> Food::Fruit