Testing a variable scope association - ruby-on-rails

Question

I'm trying to make my tests robust and really solid, and I've been breaking down some complex queries and associations into smaller ones, or refactoring and moving the data into scopes.
Given the following classes:
class Item < ActiveRecord::Base
belongs_to :location
scope :in_location, ->(location) { where(location: location) }
scope :findable, ->(location, not_ids) {
in_location(location).where.not(id: not_ids)
}
end
class Container < ActiveRecord::Base
belongs_to :location
# THIS IS WHAT I WANT TO TEST
has_many :findable_items, ->(container) {
findable(container.location, container.not_findable_ids)
}, class_name: 'Item'
end
How would you test a variable has_many relationship like this without hitting the database to a significant degree? I know I can test the Item.findable method on it's own; what I'm interested in is the container.findable_items method.
Note: the actual association being tested is more complex than this, and would require pretty extensive set-up; it's running through a few other nested associations and scopes. I'd like to avoid this setup if possible, and just test that the scope is called with the correct values.
Relevant parts of my Gemfile:
rails (4.2.3)
shoulda-matchers (2.6.2)
factory_girl (4.5.0)
factory_girl_rails (4.5.0)
rspec-core (3.3.2)
rspec-expectations (3.3.1)
rspec-its (1.2.0)
rspec-mocks (3.3.2)
rspec-rails (3.3.3)
I have shoulda-matchers in my project, so I can do the basic sanity test:
it { should have_many(:findable_items).class_name('Item') }
but this fails:
describe 'findable_line_items' do
let(:container) { #container } # where container is a valid but unsaved Container
let(:location) { #container.location }
it 'gets items that are in the location and not excluded' do
container.not_findable_ids = [1,2]
# so it doesn't hit the database
expect(Item).to receive(:findable).with(location, container.not_findable_ids)
container.findable_items
end
end
This spec fails with the following error:
1) Container findable_line_items gets items that are in the location and not excluded
Failure/Error: container.findable_items
NoMethodError:
undefined method `except' for nil:NilClass
# /[redacted]/gems/activerecord-4.2.3/lib/active_record/associations/association_scope.rb:158:in `block (2 levels) in add_constraints'
# /[redacted]/gems/activerecord-4.2.3/lib/active_record/associations/association_scope.rb:154:in `each'
# /[redacted]/gems/activerecord-4.2.3/lib/active_record/associations/association_scope.rb:154:in `block in add_constraints'
# /[redacted]/gems/activerecord-4.2.3/lib/active_record/associations/association_scope.rb:141:in `each'
# /[redacted]/gems/activerecord-4.2.3/lib/active_record/associations/association_scope.rb:141:in `each_with_index'
# /[redacted]/activerecord-4.2.3/lib/active_record/associations/association_scope.rb:141:in `add_constraints'
# /[redacted]/activerecord-4.2.3/lib/active_record/associations/association_scope.rb:39:in `scope'
# /[redacted]/gems/activerecord-4.2.3/lib/active_record/associations/association_scope.rb:5:in `scope'
# /[redacted]/gems/activerecord-4.2.3/lib/active_record/associations/association.rb:97:in `association_scope'
# /[redacted]/gems/activerecord-4.2.3/lib/active_record/associations/association.rb:86:in `scope'
# /[redacted]/gems/activerecord-4.2.3/lib/active_record/associations/collection_association.rb:423:in `scope'
# /[redacted]/gems/activerecord-4.2.3/lib/active_record/associations/collection_proxy.rb:37:in `initialize'
# /[redacted]/gems/activerecord-4.2.3/lib/active_record/relation/delegation.rb:106:in `new'
# /[redacted]/gems/activerecord-4.2.3/lib/active_record/relation/delegation.rb:106:in `create'
# /[redacted]/gems/activerecord-4.2.3/lib/active_record/associations/collection_association.rb:39:in `reader'
# /[redacted]/gems/activerecord-4.2.3/lib/active_record/associations/builder/association.rb:115:in `pickable_items'
# ./spec/models/container_spec.rb:25:in `block (3 levels) in <top (required)>'
How would you get this spec to pass, without actually setting up an Item that meets all the requirements?

Answer 1

I ended up going with a solution like this:
describe 'findable_line_items' do
let(:container) { #container } # where container is a valid but unsaved Container
let(:location) { #container.location }
it 'gets items that are in the location and not excluded' do
# so it doesn't hit the database
expect(Item).to receive(:findable).with(location, container.not_findable_ids).and_call_original
expect(container).to receive(:location).and_call_original
expect(container).to receive(:not_findable_ids).and_call_original
container.findable_items
end
end
The error that was occurring was somewhere in the ActiveRecord association setup; it was trying to instantiate an ActiveRecord array on a nil object which was being returned from my Item stub. Adding .and_call_original solved that error.
I don't really care to test that the correct objects are being returned from this association, since that scope is being tested elsewhere, just that the scope is being used. It still hits the database in this scenario, but not the 15 times that would be required to set up a full test.