fix(query): shallow clone $or, $and if merging onto empty query filter

Fix #14567

lib/query.js

@@ -2380,13 +2380,21 @@ Query.prototype.merge = function(source) {
   }
 
   opts.omit = {};
-  if (this._conditions && this._conditions.$and && source.$and) {
+  if (source.$and) {
     opts.omit['$and'] = true;
-    this._conditions.$and = this._conditions.$and.concat(source.$and);
+    if (this._conditions && this._conditions.$and) {
+      this._conditions.$and = this._conditions.$and.concat(source.$and);
+    } else {
+      this._conditions.$and = [...source.$and];
+    }
   }
-  if (this._conditions && this._conditions.$or && source.$or) {
+  if (source.$or) {
     opts.omit['$or'] = true;
-    this._conditions.$or = this._conditions.$or.concat(source.$or);
+    if (this._conditions && this._conditions.$or) {
+      this._conditions.$or = this._conditions.$or.concat(source.$or);
+    } else {
+      this._conditions.$or = [...source.$or];
+    }
   }
 
   // plain object

test/query.test.js

@@ -4054,6 +4054,25 @@ describe('Query', function() {
     });
   });
 
+  it('shallow clones $and, $or if merging with empty filter (gh-14567) (gh-12944)', function() {
+    const TestModel = db.model(
+      'Test',
+      Schema({ name: String, age: Number, active: Boolean })
+    );
+
+    let originalQuery = { $and: [{ active: true }] };
+    let q = TestModel.countDocuments(originalQuery)
+      .and([{ age: { $gte: 18 } }]);
+    assert.deepStrictEqual(originalQuery, { $and: [{ active: true }] });
+    assert.deepStrictEqual(q.getFilter(), { $and: [{ active: true }, { age: { $gte: 18 } }] });
+
+    originalQuery = { $or: [{ active: true }] };
+    q = TestModel.countDocuments(originalQuery)
+      .or([{ age: { $gte: 18 } }]);
+    assert.deepStrictEqual(originalQuery, { $or: [{ active: true }] });
+    assert.deepStrictEqual(q.getFilter(), { $or: [{ active: true }, { age: { $gte: 18 } }] });
+  });
+
   it('should avoid sending empty session to MongoDB server (gh-13052)', async function() {
     const m = new mongoose.Mongoose();