How to count child elements in angularfire?

问题

Hi I am using angularfire for my app I am listing my service and controller here. I am planning to get the count of comments which has been added to a particular post. this is my controller

    app.controller('PostViewCtrl', function ($scope,                          
     FIREBASE_URL,$routeParams,  
     Post, Auth, $timeout,$interval,$http, $rootScope, $firebase) {

    var ref = new Firebase(FIREBASE_URL);
    $scope.post = Post.get($routeParams.postId);
    $scope.comments = Post.comments($routeParams.postId);


      $scope.user = Auth.user;
      $scope.signedIn = Auth.signedIn;


            $scope.addComment = function () {
         if(!$scope.commentText || $scope.commentText === '') {
         return;
           }
           var Commentcreatedtime = moment().format('llll');
           $scope.CommentcreatedTime = Commentcreatedtime;
      var comment = {
  createdTime:  $scope.CommentcreatedTime,
  text: $scope.commentText,
  creator: $scope.user.profile.username,
  creatorUID: $scope.user.uid,
  creatorpic: $scope.user.profile.userpic,
  commentimage: $scope.object.image.info.uuid
};


$scope.comments.$add(comment);




$scope.commentText = '';
$scope.object.image.info.uuid = '';
   };

    });

this is my service

   'use strict';

   app.factory('Post', function($firebase,FIREBASE_URL){

   var ref = new Firebase(FIREBASE_URL);
    var posts = $firebase(ref.child('posts')).$asArray();

     var Post = {
    all: posts,
    create: function(post){
    return posts.$add(post).then(function(postRef){
        $firebase(ref.child('user_posts').child(post.creatorUID))
                    .$push(postRef.name());
        return postRef;
     });
      },
     get: function(postId){
    return $firebase(ref.child('posts').child(postId)).$asObject();

     },
      delete: function(post){
    return posts.$remove(post);
       },
   comments: function(postId){

     return $firebase(ref.child('comments').child(postId)).$asArray();
       }
      };

        return Post;

      });

I have tried using transaction to update a counter on the addComment event like this

   $scope.comments.$add(comment, function(error) {
    if (!error) {
     $firebase(ref.child('comments').child(postId)
     .child('commentcount')).transaction(function (count) {
      return count + 1;
    });
     }
    });

but this does not create a child to comments-- postId nor is there a counter. Please do help me. I am new to firebase and angular and your help will be much appreciated.

回答1:

You seem to be using an older version of AngularFire. Please update to the latest version. While it probably doesn't make a difference for the problem you're posting, it will make it easier for AngularFire experts to respond (since the documentation for the latest version is easiest to find).

If I ignore everything else, this is the code where you're updating the count of the number of comments for a post.

$firebase(ref.child('comments').child(postId)
 .child('commentcount')).transaction(function (count) {
  return count + 1;
});

I don't think AngularFire wraps the transaction method. And even if it does, I would not use AngularFire for this. AngularFire provides a binding from Firebase's regular JavaScript SDK to the AngularJS front-end. Counting comments is not front-end functionality, so I would just stick to the JavaScript SDK for that. Displaying the comment count is front-end functionality, so I would bind that to the $scope with AngularFire services.

Now for the actual code:

var countRef = ref.child('comments').child(postId) .child('commentcount');
countRef.transaction(function (count) {
  return (count || 0) + 1;
});

Note that this last snippet is a pretty literal copy from the Firebase documentation for transaction. A slightly more readable version is:

var countRef = ref.child('comments').child(postId) .child('commentcount');
countRef.transaction(function (count) {
  if (!count) {
    // if count does not exist, we apparently don't have any comments yet
    count = 0;
  }
  count = count + 1;
  return count;
});

来源：https://stackoverflow.com/questions/31361993/how-to-count-child-elements-in-angularfire

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angularjs

firebase

angularfire