Support through_association with singleton.

Given a load declaration like:
```ruby
load_resource :order
load_resource through: :address, through_association: :custom_address, singleton: true
```

The `:through_assocation` option wasn't being respected in the
`:singleton` branch, thus falling back to calling `find`, but odds are
you wouldn't have an ID given it was a singleton, thus leading to an
error.

Now it uses the `:through_association` before falling back to the
`name`.

Fixes #604.

Author: cgunther

CHANGELOG.md

@@ -1,3 +1,7 @@
+## Unreleased
+
+* [#878](https://github.com/CanCanCommunity/cancancan/pull/878): Support `through_association` with `singleton`. ([@cgunther][])
+
 ## 3.6.0
 
 * [#849](https://github.com/CanCanCommunity/cancancan/pull/849): Update tests matrix. ([@coorasse][])
@@ -716,3 +720,4 @@ Please read the [guide on migrating from CanCanCan 2.x to 3.0](https://github.co
 [@MrChoclate]: https://github.com/MrChoclate
 [@pandermatt]: https://github.com/pandermatt
 [@kalsan]: https://github.com/kalsan
+[@cgunther]: https://github.com/cgunther

lib/cancan/controller_resource_finder.rb

@@ -5,8 +5,9 @@ module ControllerResourceFinder
     protected
 
     def find_resource
-      if @options[:singleton] && parent_resource.respond_to?(name)
-        parent_resource.send(name)
+      singleton_association_name = @options[:through_association] || name
+      if @options[:singleton] && parent_resource.respond_to?(singleton_association_name)
+        parent_resource.send(singleton_association_name)
       elsif @options[:find_by]
         find_resource_using_find_by
       else

spec/cancan/controller_resource_spec.rb

@@ -430,6 +430,24 @@ class Dashboard; end
       expect(controller.instance_variable_get(:@model)).to eq(model)
     end
 
+    it 'finds record through has_one association with :through_association and :singleton options' do
+      params[:id] = nil
+
+      model = Model.new
+
+      category = double(custom_model: model)
+      controller.instance_variable_set(:@category, category)
+
+      resource = CanCan::ControllerResource.new(
+        controller,
+        through: :category,
+        through_association: :custom_model,
+        singleton: true,
+      )
+      resource.load_resource
+      expect(controller.instance_variable_get(:@model)).to eq(model)
+    end
+
     it 'loads the model using a custom class' do
       model = Model.new
       allow(Model).to receive(:find).with('123') { model }