Added the limit-children sample.

README.md

@@ -32,6 +32,10 @@ This can be useful to keep tack of the number of "likes" or "followers" of a som
 
 Integrates the Google Translate API to perform automatic text translation across any number of languages. Language codes can be stored in Firebase for on the fly changes.
 
+### [Limit number of child nodes](/limit-children)
+
+Make sure to keep the number of child nodes below a certain threshold. For instance this can be useful to limit the number of lines of logs or chat history below a given number.
+
 ### [Full-text search via Algolia](/fulltext-search) TODO import from go/hearth-codesamples
 
 ...

limit-children/README.md

@@ -0,0 +1,43 @@
+# Limit number of child nodes.
+
+This template shows how to keep the number of child nodes in a Firebase database below a given number. This can be used to limit the number of lines of a chat history or logs.
+
+## Cloud Function Code
+
+See file [index.js](index.js) for the code.
+
+The dependencies are listed in [package.json](package.json).
+
+## Sample Database Structure
+
+As an example we'll be using a simple chat database structure:
+
+```
+/functions-project-12345
+    /chat
+        /key-123456
+            user: "Mat",
+            text: "Hey Bob!"
+        /key-123457
+            user: "Bob",
+            text: "Hey Mat! What's Up?"
+```
+
+## Trigger rules
+
+Below is the trigger rule for the `truncate` function making sure it's triggered when a new message is added.
+
+```
+  "functions": {
+    ".source": "functions",
+    "truncate": {
+      "triggers": {
+        "database": {
+          "path": "/chat/$messageid"
+        }
+      }
+    }
+  }
+```
+
+

limit-children/index.js

@@ -0,0 +1,49 @@
+/**
+ * Copyright 2015 Google Inc. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+'use strict';
+
+// Create an all access Firebase Database reference.
+var Firebase = require('firebase');
+var env = require('./env');
+var ref = new Firebase(env.get('firebase.database.url'), 'admin');
+ref.auth(env.get('firebase.database.token'));
+
+// Max number of lines of the chat history.
+var MAX_LOG_COUNT = 5;
+
+// Removes siblings of the node that element that triggered the function if there are more than MAX_LOG_COUNT
+function truncate(context, data) {
+  var parentRef = ref.child(data.path).parent();
+  parentRef.once('value', function(snapshot) {
+    if (snapshot.numChildren() > MAX_LOG_COUNT) {
+      var childCount = 0;
+      var updates = {};
+      snapshot.forEach(function (child) {
+        if (++childCount < snapshot.numChildren() - MAX_LOG_COUNT) {
+          updates[child.key()] = null;
+        }
+      });
+      // Update the parent. This effectiovely removes the extra children.
+      parentRef.update(updates, context.done);
+    } else {
+      context.done();
+    }
+  });
+}
+
+module.exports = {
+  truncate: truncate
+};

limit-children/package.json

@@ -0,0 +1,6 @@
+{
+  "main": "index.js",
+  "dependencies": {
+    "firebase": "^2.4.0"
+  }
+}