set Document.modelName in constructor

fix Automattic#12669

lib/document.js

@@ -195,6 +195,10 @@ function Document(obj, fields, skipId, options) {
     });
   }
 
+  if (this.modelName === undefined) {
+    this.modelName = this.constructor.modelName;
+  }
+
   applyQueue(this);
 }
 

package.json

@@ -94,7 +94,7 @@
     "release": "git pull && git push origin master --tags && npm publish",
     "release-legacy": "git pull origin 5.x && git push origin 5.x --tags && npm publish --tag legacy",
     "mongo": "node ./tools/repl.js",
-    "test": "mocha --exit ./test/*.test.js",
+    "test": "mocha --exit --timeout 30000 ./test/*.test.js",
     "test-rs": "START_REPLICA_SET=1 mocha --timeout 30000 --exit ./test/*.test.js",
     "test-tsd": "node ./test/types/check-types-filename && tsd",
     "tdd": "mocha ./test/*.test.js --inspect --watch --recursive --watch-files ./**/*.{js,ts}",

test/document.test.js

@@ -11996,6 +11996,13 @@ describe('document', function() {
       title: 'The power of JavaScript'
     });
   });
+
+  it('should apply `modelName` property to new document (gh-12669)', async function() {
+    const Model = db.model('Test', new Schema());
+
+    const doc = new Model();
+    assert.equal(doc.modelName, 'Test');
+  });
 });
 
 describe('Check if instance function that is supplied in schema option is availabe', function() {