Relationship field not working in Trigger.New/Trigger.Old

While  i was working on a trigger i observed one strange behaviour. Lets say there is one custom object "Test__c" which has a look up to Account and the field being 'Account__c'. I performed update operation on Test__c so here is how my trigger would look like
<br /> if(Trigger.isupdate)<br /> {<br /> for(Test__c tempTest:Trigger.New)<br /> {<br />    String temp=tempTest.Account__r.Name;  //This dint work<br /> //Some code here<br />  }<br /> }<br />
Here though i was sure Name field of Account contains some value the same is not assigned to the string. I tried it many times but always ended up no where with no value being assigned.So instead of directly getting the value from Trigger.New i got the ids of "Test__c" records and then separately queried them to get respective values and to my surprise it worked..
<br /> <br /> if(Trigger.isupdate)<br /> {<br /> set<Id> testIds = new set<Ids>();<br /> for(Test__c tempTest:Trigger.New)<br /> {<br />    testIds.add(tempTest.id);<br /> //Some code here<br />  }<br /> }<br /> <br /> for(Test__c tempTest1 :[select Account__c,Account__r.Name from Test__c where id in :testIds])<br /> {<br />     String temp=tempTest1.Account__r.Name  //This worked<br /> }<br />

Though strange i dint understand the underlying concept (Totally Dark!!!) .Can some one beam some light on this !!


H C !!