Q: Emberjs how can I make collection arrangedContent and searchResults work together? I have a controller that observes a search field like so:
Scrolls.IndexController = Ember.ArrayController.extend({    
  searchResult: function() {
    var that = this;
    this.get('model').set('content', this.store.filter('scroll', function(item) {
      var searchTerm = that.get('searchCard');
      var regExp = new RegExp(searchTerm, 'i');
      return regExp.test(item.get('name'));
    }));
  }.observes('searchCard')
});

Which works great, but once I add a method that overrides arrangedContent to limit the returned items, it stops re-rendering.
Scrolls.IndexController = Ember.ArrayController.extend({
  arrangedContent: Ember.computed('content', function() {
    var count = 0;
    return this.get('content').filter(function() {
      count++;
      return count <= 3;
    });
  }),

  searchResult: function() {
    var that = this;
    this.get('model').set('content', this.store.filter('scroll', function(item) {
      var searchTerm = that.get('searchCard');
      var regExp = new RegExp(searchTerm, 'i');
      return regExp.test(item.get('name'));
    }));
  }.observes('searchCard')
});

How can I get make what I'm doing to behave nicely with each other?

A: I see a few things here that jump out to me. First one being, in the context of a controller, content and model are the same thing so in the observer, when you do:

this.get('model').set('content'

You're setting a property of 'content' on the model when I think you actually intend to set the content directly on the controller, like this:

this.set('content',

I also kind of wonder whether you really need to override the content and arrangedContent properties (not sure what the calling code looks like). I suspect that might cause some bugs later. Instead, I wonder if you could set it up like this:
Scrolls.IndexController = Ember.ArrayController.extend({
  firstThreeSearchResults: function() {
    var count = 0;
    return this.get('searchResults').filter(function() {
      count++;
      return count <= 3;
    });
  }.property('searchResults'),

  searchResults: function() {
    var searchTerm = this.get('searchCard');
    return this.store.filter('scroll', function(item) {
      var regExp = new RegExp(searchTerm, 'i');
      return regExp.test(item.get('name'));
    });
  }.property('searchCard')
});

Final possible problem is the use of the filter function called on the store. According to the docs, this function: "returns a live RecordArray that remains up to date as new records are loaded into the store or created locally." The problem being, though the filter might update as new results are added, it might not cause the computed property that looks for the first three results to update. That is, the binding on that computed property might not fire. One way to get around this would be to do something like this:
Scrolls.IndexRoute = Ember.Route.extend({
  model: function() {
    return this.store.find();
  }
});

Scrolls.IndexController = Ember.ArrayController.extend({
  firstThreeSearchResults: function() {
    var count = 0;
    return this.get('searchResults').filter(function() {
      count++;
      return count <= 3;
    });
  }.property('searchResults'),

  searchResults: function() {
    var searchTerm = this.get('searchCard');
    return this.get('content').filter(function(item) {
      var regExp = new RegExp(searchTerm, 'i');
      return regExp.test(item.get('name'));
    });
  }.property('searchCard', 'content.length')
});

