Allow duplicate objects in ngRepeat.

[spamdaemon]

Currently, ngRepeat doesn't allow modification of a list that contains
duplicates. For example, while ng-repeat='i in [1,1,1]' is allowed, it
is not possible to add a 1 to the list after it has been iterated at
least once. This patch addresses this issue.
Just by observing a change from [1,1,1] to [1,1,1,1] it is not possible
to know the position where the additional 1 was inserted (vice versa for
removal of 1). For the purposes of this patch, we assume that the item
was inserted after all other duplicates (or is the last duplicate to be
removed).
This patch does have a visible effect when using animations. Consider
the following list [a,b,a] and inserting a in position 0 to yield
[a,a,b,a]. Since we're assuming that a was added, we need to apply a
series of moves to yield the desired list:

• a in position 0 does not change
• a moves from position 2 to position 1
• b moves from position 1 to position 2
• a new a is added in position 3
  It is probably possible to minimize the number of moves necessary, but that
  would come at some cost.
  This patch assumes that no two objects that are not occupying the same
  memory location will have the same $$hashKey (which is the same
  assumption of the previous implementation but it was not stated!)

[mhevery]

This works as intended. If there is a need for duplicate entries then we can no longer keep association between the DOM and the entry and so the only way to associate is by position.

ng-repeat='i in [1,1,1] track by $index' will do what you want.

Closing the issue.

[IgorMinar]

+1 for misko's reply

[spamdaemon]

Ok that works great.I did not see this in the documentation.

[aaronshaf]

Please put this in the in the documentation:

http://docs.angularjs.org/api/ng.directive:ngRepeat

[zakdances]

When I use @mhevery's example, all I get is

Syntax Error: Token 'track' is an unexpected token at column 10 of the expression [[entries]track by $index] starting at [track by $index].

This is my html attribute:

ng-repeat="entry in [entries]track by $index"

[KevinBrogan]

@zakdances you're missing a space between [entries] and track

[estufa8]

same problem occurs using (key,value) with duplicated values, like in
ng-repeat="(k,v) in {a:0, b:0}"
track by $index does the trick, thanks

[nickretallack]

Why is this still undocumented?

[iomico]

Please add it to the documentation.

[pkozlowski-opensource]

@nickretallack @guilloche It is documented:
http://code.angularjs.org/1.1.5/docs/api/ng.directive:ngRepeat

Docs at the http://docs.angularjs.org/ point to the latest stable version (1.0.x). If you need docs for the unstable branch (1.1.x) go to http://code.angularjs.org/

[iomico]

Thanks a lot for the explanation.

[tamtakoe]

I want use ng-repeat without duplicates. This behaviour is prefer for me, but I dont want see error message. Can I disable it?

What about perfomance when ng-repeat delete duplicates?

[netpoetica]

This is a major headache, really. Even the documentation in the new version simply does not make any sense. A code example for this text example would be helpful.

http://code.angularjs.org/1.1.5/docs/api/ng.directive:ngRepeat

"For example: item in items track by $id(item). A built in $id() function can be used to assign a unique $$hashKey property to each item in the array. This property is then used as a key to associated DOM elements with the corresponding item in the array by identity. Moving the same object in array would move the DOM element in the same way ian the DOM."

What is "$id" supposed to be? It's a "function" but with a name of an internal Angular item "$id", so it looks like $id is a provided helper method, but I doubt it. So I'm imaginging maybe this is a $scope.$id = function(){//do it yourself, we dont care }); type of thing, but due to the fact that 1 sentence into the documentation, I already don't know what they're trying to do, going any further is simply impossible - at least for me anyway. Maybe it's just me.

More importantly, this sounds like some kind of problem that someone on the Angular dev team would understand really well because they've fixed bugs on it, they understand how it effects core tracking mechanisms. However, someone who doesn't have that level of detailed understanding, has absolutely no idea what problem you are trying to solve by making it impossible to do something that makes so much sense.

[netpoetica]

Maybe I am doing something wrong, but see this Plunkr for an example of why this duplicates maybe should be allowed: http://plnkr.co/edit/km4uoNPRjh624QHgEeeB?p=preview

This is the result of implementing from the docs, "track as $id($index)". If you click "add" to add a new item to your array, and then select an item, and then click "add" again - BOOM, both items reset. Why? - if you remove the "track by" from the Stuff select, you will get duplicates error.

It seems to me like Angular doesn't like that the select list has the same option values in both ng-options within the ng-repeat. As you can see there, the important thing is that Angular knows to set the value of my $scope view model property to the selected option - the select options should not even be tracked by angular - they do not change. The selected state changes, but that is it.

I will post this to the Google group as well for an answer - I am posting here to illustrate a point, which is mostly the point that I have spent the last 2 days trying to wrap my ahead around a silly error that looks, again, like maybe the angular team is aware of why we "shouldn't" have duplicates, but it certainly doesn't make sense to me - and quite a few other people if you do a search for "Duplicates not allowed"

See Google group post: https://groups.google.com/forum/#!topic/angular/Biy9jjsyHNM

Much confusion around this topic. Some people are seeing results with track by $id($index) but I think my above example shows that this issue is not solved.

EDIT: Trying to bind to the item in the loop seems to create a closure issue where all items bind to the same model, even though technically it should be a different reference in every iteration. If you instead specify to bind ng-model to the parent element by index: http://plnkr.co/edit/1CmulrjigYQYjmfewUcJ?p=preview

[nickretallack]

In the current stable version of angular you can put duplicate items in an ng-repeat just fine. Why did we break this feature?

<div ng-app><div ng-repeat="item in [1,1,1]">{{item}}</div></div>

I'm afraid if this breaking change ever goes into the stable version it is going to cause unexpected problems for people.

Imagine this situation: A developer writes an app with an ng-repeat that usually doesn't contain duplicates. However, in rare situations, it does. They release their web app. Later, users report that lists disappear from the page sometimes. Imagine being this developer and wondering what went wrong.

[mgan59]

I just had a painful hour trying to figure out what was wrong. May want to get the current docs - http://docs.angularjs.org/api/ng.directive:ngRepeat updated for those of us that are new to angular.