loop over value keys to check wich value need to be updated

firebase-database-behavior.html

@@ -90,9 +90,10 @@
       },
 
       __pathChanged: function(path, oldPath) {
-        if (oldPath != null && !this.disabled && this.__pathReady(path)) {
+        if (!this.disabled && !this.valueIsEmpty(this.data)) {
           this.syncToMemory(function() {
             this.data = this.zeroValue;
+            this.__needSetData = true;
           });
         }
       },

firebase-document.html

@@ -39,6 +39,7 @@
     ],
 
     attached: function() {
+      this.__needSetData = true;
       this.__refChanged(this.ref, this.ref);
     },
 
@@ -184,13 +185,30 @@
 
       if (value == null) {
         value = this.zeroValue;
+        this.__needSetData = true;
       }
 
-      if (!this.new) {
+      if (!this.isNew) {
         this.async(function() {
           this.syncToMemory(function() {
             this._log('Updating data from Firebase value:', value);
-            this.set('data', value);
+
+            // set the value if: 
+            // it is the first time we run this (or the path has changed and we are back with zeroValue)
+            // or if  this.data does not exist 
+            // or value is primitive 
+            // or if firebase value obj contain less keys than this.data (https://github.com/Polymer/polymer/issues/2565)
+            if (this.__needSetData || !this.data || typeof value !== 'object' || ( Object.keys(value).length <  Object.keys(this.data).length)) {
+              this.__needSetData = false;
+              return this.set('data', value);
+            }
+
+            // now, we loop over keys
+            for (var prop in value) {
+              if(value[prop] !== this.data[prop]) {
+                this.set(['data', prop], value[prop]);
+              }
+            }
           });
         });
       }