perf(schema): clear childSchemas when overwriting existing path to avoid performance degradations

Fix #15253

Author: vkarpov15

lib/schema.js

@@ -1118,6 +1118,9 @@ Schema.prototype.path = function(path, obj) {
   this.paths[path] = this.interpretAsType(path, obj, this.options);
   const schemaType = this.paths[path];
 
+  // If overwriting an existing path, make sure to clear the childSchemas
+  this.childSchemas = this.childSchemas.filter(childSchema => childSchema.path !== path);
+
   if (schemaType.$isSchemaMap) {
     // Maps can have arbitrary keys, so `$*` is internal shorthand for "any key"
     // The '$' is to imply this path should never be stored in MongoDB so we

test/schema.test.js

@@ -3889,4 +3889,34 @@ describe('schema', function() {
       assert.throws(() => schema.toJSONSchema(), /unsupported SchemaType to JSON Schema: Mixed/);
     });
   });
+
+  it('path() clears existing child schemas (gh-15253)', async function() {
+    const RecursiveSchema = new mongoose.Schema({
+      data: String
+    });
+
+    const s = [RecursiveSchema];
+    RecursiveSchema.path('nested', s);
+    assert.strictEqual(RecursiveSchema.childSchemas.length, 1);
+    RecursiveSchema.path('nested', s);
+    assert.strictEqual(RecursiveSchema.childSchemas.length, 1);
+    RecursiveSchema.path('nested', s);
+    assert.strictEqual(RecursiveSchema.childSchemas.length, 1);
+    RecursiveSchema.path('nested', s);
+    assert.strictEqual(RecursiveSchema.childSchemas.length, 1);
+
+    const generateRecursiveDocument = (depth, curr = 0) => {
+      return {
+        name: `Document of depth ${curr}`,
+        nested: depth > 0 ? new Array(3).fill().map(() => generateRecursiveDocument(depth - 1, curr + 1)) : [],
+        data: Math.random()
+      };
+    };
+
+    const TestModel = db.model('Test', RecursiveSchema);
+    const data = generateRecursiveDocument(6);
+    const doc = new TestModel(data);
+    await doc.save();
+
+  });
 });