Throw error when setting authData to null (#6154)

* added ignore authData field

* add fix for Postgres

* add test for mongoDB

* add test login with provider despite invalid authData

* removed fit

* fixed ignoring authData in postgres

* Fix postgres test

* Throw error instead of ignore

* improve tests

* Add mongo test

* allow authData when not user class

* fix tests

* more tests

* add condition to synthesize authData field only in _User class

it is forbidden to add a custom field name beginning with `_`, so if the object is not `_User` , the transform should throw

* add warning log when ignoring invalid `authData` in `_User`

* add test to throw when custom field begins with underscore

spec/MongoTransform.spec.js

@@ -479,6 +479,46 @@ describe('parseObjectToMongoObjectForCreate', () => {
     }).toThrow();
     done();
   });
+
+  it('ignores User authData field in DB so it can be synthesized in code', done => {
+    const input = {
+      _id: '123',
+      _auth_data_acme: { id: 'abc' },
+      authData: null,
+    };
+    const output = transform.mongoObjectToParseObject('_User', input, {
+      fields: {},
+    });
+    expect(output.authData.acme.id).toBe('abc');
+    done();
+  });
+
+  it('can set authData when not User class', done => {
+    const input = {
+      _id: '123',
+      authData: 'random',
+    };
+    const output = transform.mongoObjectToParseObject('TestObject', input, {
+      fields: {},
+    });
+    expect(output.authData).toBe('random');
+    done();
+  });
+});
+
+it('cannot have a custom field name beginning with underscore', done => {
+  const input = {
+    _id: '123',
+    _thisFieldNameIs: 'invalid',
+  };
+  try {
+    transform.mongoObjectToParseObject('TestObject', input, {
+      fields: {},
+    });
+  } catch (e) {
+    expect(e).toBeDefined();
+  }
+  done();
 });
 
 describe('transformUpdate', () => {