MONGOID-5839 Fix eager-loading from STI subclasses (backport to 8.0)

(backport to 8.0)

If the root of a query is an STI subclass (e.g. Subclass.all) AND
the query tries to eager load (includes) another association, the
eager load was failing because it was looking for inverse
associations using the STI subclass name, instead of the class at
the root of the hierarchy.

lib/mongoid/association/eager_loadable.rb

@@ -31,6 +31,9 @@ def preload(associations, docs)
         docs_map = {}
         queue = [ klass.to_s ]
 
+        # account for single-collection inheritance
+        queue.push(klass.root_class.to_s) if klass != klass.root_class
+
         while klass = queue.shift
           if as = assoc_map.delete(klass)
             as.each do |assoc|

lib/mongoid/traversable.rb

@@ -300,6 +300,18 @@ def hereditary?
         !!(Mongoid::Document > superclass)
       end
 
+      # Returns the root class of the STI tree that the current
+      # class participates in. If the class is not an STI subclass, this
+      # returns the class itself.
+      #
+      # @return [ Mongoid::Document ] the root of the STI tree
+      def root_class
+        root = self
+        root = root.superclass while root.hereditary?
+
+        root
+      end
+
       # When inheriting, we want to copy the fields from the parent class and
       # set the on the child to start, mimicking the behavior of the old
       # class_inheritable_accessor that was deprecated in Rails edge.

spec/mongoid/association/eager_spec.rb

@@ -14,14 +14,36 @@
       Mongoid::Contextual::Mongo.new(criteria)
     end
 
+    let(:association_host) { Account }
+
     let(:inclusions) do
       includes.map do |key|
-        Account.reflect_on_association(key)
+        association_host.reflect_on_association(key)
       end
     end
 
     let(:doc) { criteria.first }
 
+    context 'when root is an STI subclass' do
+      # Driver has_one Vehicle
+      # Vehicle belongs_to Driver
+      # Truck is a Vehicle
+
+      before do
+        Driver.create!(vehicle: Truck.new)
+      end
+
+      let(:criteria) { Truck.all }
+      let(:includes) { %i[ driver ] }
+      let(:association_host) { Truck }
+
+      it 'preloads the driver' do
+        expect(doc.ivar(:driver)).to be false
+        context.preload(inclusions, [ doc ])
+        expect(doc.ivar(:driver)).to be == Driver.first
+      end
+    end
+
     context "when belongs_to" do
 
       let!(:account) do
@@ -42,7 +64,7 @@
       it "preloads the parent" do
         expect(doc.ivar(:person)).to be false
         context.preload(inclusions, [doc])
-        expect(doc.ivar(:person)).to eq(doc.person)
+        expect(doc.ivar(:person)).to be == person
       end
     end