The LastModifiedDate Field - Behavior Differences in FeedItem vs. Other sObjects

TL;DR - FeedItem.LastModifiedDate does not behave the same way as a normal sObject's LastModifiedDate in, say, a master-detail relationship.


Reading through the Spring '11 release notes (PDF) in preparation for my SF 501 Cert. exam next week, I discovered an oddity. As you can see under the "Changed Chatter Objects" section on page 76, the LastModifiedDate field has been added to Chatter objects. This is great, but the definition of its behavior is what caught my interest. Here's what they say:

When a feed item is created, LastModifiedDate is the same as CreatedDate. If a FeedComment is inserted onthat feed item, then LastModifiedDate becomes the CreatedDate for that FeedComment. Deleting theFeedComment does not change the LastModifiedDate.

Woah, so the LastModifiedDate for a FeedItem changes even when you don't directly interact with that FeedItem? Not the behavior that I would expect to see. Now, I can see this as useful for Chatter feeds because the time of update is the important bit of information here, but isn't this an exception to the rule? That is, if I create two sObjects, one called 'Master' and one called 'Detail' with a master-detail relationship between the two, will the addition of a new Detail record update the LastModifiedDate of the Master record?

I would guess that it wouldn't, but I couldn't find any documentation on it, so I went to my personal dev org to test it out.

1. I defined a 'Master' object and a 'Child' object with a master-detail relationship between them.
2. I created a new 'Master' record.
1. (Master's LastModifiedDate == 6/13/2011 2:45 PM)
3. I created a new 'Detail' record on that 'Master' record.
1. (Detail's LastModifiedDate == 6/13/2011 2:47 PM)
2. (Master's LastModifiedDate == 6/13/2011 2:45 PM)

According to my results, adding a detail record to a master record will not change the master record's LastModifiedDate. Chatter's FeedItem.LastModifiedDate is the exception, so keep this in mind.