fix(discriminator): shallow clone Schema.prototype.obj before merging schemas to avoid modifying original obj

Fix #14827

Author: vkarpov15

lib/helpers/model/discriminator.js

@@ -115,6 +115,9 @@ module.exports = function discriminator(model, name, schema, tiedValue, applyPlu
       }
     }
 
+    // Shallow clone `obj` so we can add additional properties without modifying original
+    // schema. `Schema.prototype.clone()` copies `obj` by reference, no cloning.
+    schema.obj = { ...schema.obj };
     mergeDiscriminatorSchema(schema, baseSchema);
 
     // Clean up conflicting paths _after_ merging re: gh-6076

test/model.discriminator.test.js

@@ -620,6 +620,23 @@ describe('model', function() {
         Person.discriminator('Parent2', parentSchema.clone());
       });
 
+      it('clone() does not modify original schema `obj` (gh-14821)', function() {
+        const personSchema = new Schema({
+          name: String
+        }, { discriminatorKey: 'kind' });
+
+        const parentSchema = new Schema({
+          child: String
+        });
+
+        const Person = db.model('Person', personSchema);
+        const Parent = Person.discriminator('Parent', parentSchema.clone());
+
+        assert.ok(!Person.schema.obj.child);
+        assert.ok(!personSchema.obj.child);
+        assert.ok(Parent.schema.obj.child);
+      });
+
       it('clone() allows reusing with different models (gh-5721)', async function() {
         const schema = new mongoose.Schema({
           name: String