Fundamental principle about data management in Flex

Question

I'm not event sure if my title really reflect what's going on here... I just realized that apparently I'm missing a fundamental principle in the way Flex is handling data (memory?) and I need some help to understand it.

I have the a MySQL table named friends that contains the following rows ;

MySQL table friends
+-----------+----------+
| firstname | lastname |
+-----------+----------+
| Mark      | Smith    |
| Andrew    | Barnes   |
+-----------+----------+

In the next example, I'm calling my friendsService.getAllFriends() three times and then, I assign the result to the DataProvider of three different DataGrids.

The second time I send a query to the database, I'm changing the firstname of Mark to Peter, only before assigning the result to the DataProvider. I'm not updating the database in any way, right? :-)

What I expect :

MyDataGrid1
+-----------+----------+
| firstname | lastname |
+-----------+----------+
| Mark      | Smith    |
| Andrew    | Barnes   |
+-----------+----------+

MyDataGrid2
+-----------+----------+
| firstname | lastname |
+-----------+----------+
| Peter     | Smith    | <---- Say hello to Peter :-)
| Andrew    | Barnes   |
+-----------+----------+

MyDataGrid3
+-----------+----------+
| firstname | lastname |
+-----------+----------+
| Mark      | Smith    |
| Andrew    | Barnes   |
+-----------+----------+

After calling the second query (The one where I switch Mark to Peter programmatically), It looks like all three results are binded in some way I don't understand... I obtain :

MyDataGrid1
+-----------+----------+
| firstname | lastname |
+-----------+----------+
| Peter     | Smith    |
| Andrew    | Barnes   |
+-----------+----------+

MyDataGrid2
+-----------+----------+
| firstname | lastname |
+-----------+----------+
| Peter     | Smith    |
| Andrew    | Barnes   |
+-----------+----------+

MyDataGrid3
+-----------+----------+
| firstname | lastname |
+-----------+----------+
| Peter     | Smith    |
| Andrew    | Barnes   |
+-----------+----------+

It looks like Flex is not really asking for new data, assuming that it already have the answer from friendsService.getAllFriends() ...

Any enlightment for me? :-)

THANKS !

<?xml version="1.0" encoding="utf-8"?>
<s:WindowedApplication xmlns:fx="http://ns.adobe.com/mxml/2009" 
    xmlns:s="library://ns.adobe.com/flex/spark" 
    xmlns:mx="library://ns.adobe.com/flex/mx" 
    xmlns:friendsservice="services.friendsservice.*"
    creationComplete="creationCompleteHandler(event)">

    <fx:Script>
        <![CDATA[
            import mx.controls.Alert;
            import mx.events.FlexEvent;
            import mx.rpc.events.ResultEvent;

            private var _step:Number = 1;

            protected function creationCompleteHandler(event:FlexEvent):void
            {
                getAllFriends();
            }

            protected function getAllFriends():void
            {
                getAllFriendsResult.token = friendsService.getAllFriends();
            }

            protected function getAllFriendsResult_resultHandler(event:ResultEvent):void
            {

                if (_step == 1) {
                    assignValuesToDataGrid(MyDataGrid1, event.result, _step);
                    dbg.text = "MyDataGrid1 has been loaded";
                }

                if (_step == 2) {
                    assignValuesToDataGrid(MyDataGrid2, event.result, _step);
                    dbg.text = "MyDataGrid2 has been loaded";
                }

                if (_step == 3) {
                    assignValuesToDataGrid(MyDataGrid3, event.result, _step);
                    dbg.text = "MyDataGrid3 has been loaded";
                }

                _step++;
            }

            protected function assignValuesToDataGrid(dg:DataGrid, result:Object, step:Number):void
            {
                if (step == 2)
                    result[0].firstname = "Peter";

                dg.dataProvider = result;
            }

        ]]>
    </fx:Script>

    <fx:Declarations>
        <s:CallResponder id="getAllFriendsResult" result="getAllFriendsResult_resultHandler(event)"/>
        <friendsservice:FriendsService id="friendsService"/>
    </fx:Declarations>

    <s:layout>
        <s:VerticalLayout/>
    </s:layout>

    <s:Button label="Load next DataGrid" click="getAllFriends()"/>

    <mx:DataGrid id="MyDataGrid1" width="100%" height="100">
        <mx:columns>
            <mx:DataGridColumn headerText="First name" dataField="firstname"/>
            <mx:DataGridColumn headerText="Last name" dataField="lastname"/>
        </mx:columns>
    </mx:DataGrid>

    <mx:DataGrid id="MyDataGrid2" width="100%" height="100">
        <mx:columns>
            <mx:DataGridColumn headerText="First name" dataField="firstname"/>
            <mx:DataGridColumn headerText="Last name" dataField="lastname"/>
        </mx:columns>
    </mx:DataGrid>

    <mx:DataGrid id="MyDataGrid3" width="100%" height="100">
        <mx:columns>
            <mx:DataGridColumn headerText="First name" dataField="firstname"/>
            <mx:DataGridColumn headerText="Last name" dataField="lastname"/>
        </mx:columns>
    </mx:DataGrid>

    <s:Label id="dbg" fontWeight="bold"/>

</s:WindowedApplication>

I'm not familiar with your FriendService. Assuming this is a custom component, seeing it would help, or at least the relevant portions of it. — Sam DeHaan
@Sam DeHaan My FriendService is simply a service generated using the "Connect to Data/Service" Tool in Flash Builder 4, using data type valueObject — Jivago
Could you run a debug on the _step variable on the function assignValuesToDataGrid to check what are its values when it is being triggered three times? — Angelo
@Jivago, include the friends service class. I think there might be some sort of caching involved. — J_A_X
@Angelo the _step variable is simply incremented each time I call the function, using a simple ++ incrementation. No need to debug here, I just wanted a very simple way to illustrate the problem here. — Jivago

Johnny Everson Johnny Everson · Accepted Answer · 2011-08-01T16:19:32

The FriendService is using AbstractEntityMetadata to manage the entities. So, I think it is merging the friends based on it's IDs.

I mean, let's say Mark has id 1, since it is a managed entity, when you make a new call to the server, instead of creating new objects, it searches first for instances of matching id 1 and update it. That's why all three datagrids have the same objects.

Fundamental principle about data management in Flex

2 Answers