Fix defaultMeta override in child logger #1864

lib/winston/logger.js

@@ -44,13 +44,29 @@ class Logger extends Transform {
 
   child(defaultRequestMetadata) {
     const logger = this;
+
     return Object.create(logger, {
       write: {
         value: function (info) {
+
+          // Remark: defaultMeta is passed into the info object,
+          // and we need to check if it is there because a user
+          // specified it in a log call or it is there as a result
+          // of being passed to createLogger as defaultMeta.
+          const defaultMetaOverride = {};
+          if (logger.defaultMeta) {
+            for (const key of Object.getOwnPropertyNames(logger.defaultMeta)) {
+              if (info[key] === logger.defaultMeta[key]) {
+                defaultMetaOverride[key] = defaultRequestMetadata[key];
+              }
+            }
+          }
+
           const infoClone = Object.assign(
             {},
             defaultRequestMetadata,
-            info
+            info,
+            defaultMetaOverride
           );
 
           // Object.assign doesn't copy inherited Error

test/logger.test.js

@@ -997,12 +997,16 @@ describe('Should support child loggers & defaultMeta', () => {
     });
 
     const logger = winston.createLogger({
+      defaultMeta: {
+        requestId: '38',
+        service: 'root-service-logger'
+      },
       transports: [
         mockTransport.createMockTransport(assertFn)
       ]
     });
 
-    const childLogger = logger.child({ service: 'user-service' });
+    const childLogger = logger.child({ service: 'user-service', requestId: '154' });
     childLogger.info('dummy message', { requestId: '451' });
   });