only remove object if it becomes empty because of casting

Author: 0x0a0d

lib/cast.js

@@ -69,12 +69,15 @@ module.exports = function cast(schema, obj, options, context) {
         if (val[k] == null || typeof val[k] !== 'object') {
           throw new CastError('Object', val[k], path + '.' + k);
         }
+
+        const beforeCastKeysLength = Object.keys(val[k]).length;
         val[k] = cast(schema, val[k], options, context);
-        if (Object.keys(val[k]).length === 0) {
+        if (Object.keys(val[k]).length === 0 && beforeCastKeysLength !== 0) {
           val.splice(k, 1);
         }
       }
 
+      // delete empty: {$or: []} -> {}
       if (val.length === 0) {
         delete obj[path];
       }

test/docs/cast.test.js

@@ -125,12 +125,16 @@ describe('Cast Tutorial', function() {
       });
       Character = mongoose.model('Character', schema);
 
-      const query = Character.findOne({ $or: [{ notInSchema: { $lt: 'not a number' } }], $and: [{ name: 'abc' }, { age: { $gt: 18 } }, { notInSchema: { $lt: 'not a number' } }] });
+      const query = Character.findOne({
+        $or: [{ notInSchema: { $lt: 'not a number' } }],
+        $and: [{ name: 'abc' }, { age: { $gt: 18 } }, { notInSchema: { $lt: 'not a number' } }],
+        $nor: [{}] // should be kept
+      });
 
       await query.exec();
       query.getFilter(); // Empty object `{}`, Mongoose removes `notInSchema`
       // acquit:ignore:start
-      assert.deepEqual(query.getFilter(), { $and: [{ name: 'abc' }, { age: { $gt: 18 } }] });
+      assert.deepEqual(query.getFilter(), { $and: [{ name: 'abc' }, { age: { $gt: 18 } }], $nor: [{}] });
       // acquit:ignore:end
     });