fix: set default for dotted path projection

When a schema included a subdocument that had a default value of empty
object and that subdocument itself included some defaults, the
subdocument was always null if a query was run with a dotted path
projection to the subdocument's default values.

Check not only that the curent path being evaluated to have defaults
applied, but also check if the current path is part of the included
children in the projection.

lib/document.js

@@ -479,7 +479,7 @@ function $__applyDefaults(doc, fields, exclude, hasIncludedChildren, isBeforeSet
           break;
         }
       } else if (exclude === false && fields && !included) {
-        if (curPath in fields) {
+        if (curPath in fields || type.$isSingleNested && hasIncludedChildren[curPath]) {
           included = true;
         } else if (!hasIncludedChildren[curPath]) {
           break;

test/document.test.js

@@ -10998,4 +10998,66 @@ describe('document', function() {
     assert.equal(typeof doc.somethingElse, 'string');
     delete String.type;
   });
+
+  it('applies subdocument defaults when projecting dotted subdocument fields', async function() {
+    const grandChildSchema = new mongoose.Schema({
+      name: {
+        type: mongoose.Schema.Types.String,
+        default: () => 'grandchild'
+      }
+    });
+
+    const childSchema = new mongoose.Schema({
+      name: {
+        type: mongoose.Schema.Types.String,
+        default: () => 'child'
+      },
+      grandChild: {
+        type: grandChildSchema,
+        default: () => ({})
+      }
+    });
+
+    const parentSchema = new mongoose.Schema({
+      name: mongoose.Schema.Types.String,
+      child: {
+        type: childSchema,
+        default: () => ({})
+      }
+    });
+
+    const ParentModel = db.model('Parent', parentSchema);
+    // insert an object without mongoose adding missing defaults
+    const result = await db.collection('Parent').insertOne({ name: 'parent' });
+
+    // ensure that the defaults are populated when no projections are used
+    const doc = await ParentModel.findById(result.insertedId).exec();
+    assert.equal(doc.name, 'parent');
+    assert.equal(doc.child.name, 'child');
+    assert.equal(doc.child.grandChild.name, 'grandchild');
+
+    // ensure that defaults are populated when using an object projection
+    const projectedDoc = await ParentModel.findById(result.insertedId, {
+      name: 1,
+      child: {
+        name: 1,
+        grandChild: {
+          name: 1
+        }
+      }
+    }).exec();
+    assert.equal(projectedDoc.name, 'parent');
+    assert.equal(projectedDoc.child.name, 'child');
+    assert.equal(projectedDoc.child.grandChild.name, 'grandchild');
+
+    // ensure that defaults are populated when using dotted path projections
+    const dottedProjectedDoc = await ParentModel.findById(result.insertedId, {
+      name: 1,
+      'child.name': 1,
+      'child.grandChild.name': 1
+    }).exec();
+    assert.equal(dottedProjectedDoc.name, 'parent');
+    assert.equal(dottedProjectedDoc.child.name, 'child');
+    assert.equal(dottedProjectedDoc.child.grandChild.name, 'grandchild');
+  });
 });