Problem

The last Chatter date is not easy to get to when writing reports, formulas, or custom UIs. If only there was a field on the sObject that had the last Chatter date that automatically updates when users post on a feed or comment on a post.

Solution

Build a trigger that saves the last Chatter date on the parent sObject if it has a last chatter date field. Architecture: The ChatterDate class is a small class with one static method called updateChatterTimestamp. The static method is called by a Feed Item and Feed Comment trigger with one parameter, the trigger.newMap. The ChatterDate class is generic so all you need to do is add the LastChatterDate__c field to an sObject that is Chatter enabled.

public class ChatterDate {

    private static final String LAST_CHAT_DATE_FIELD_NAME = 'LastChatterDate__c';
    
    public static void updateChatterTimestamp(map<Id, Sobject> sobjectMap) {
        try {
            // Get the global schema
            map<String, Schema.Sobjecttype> globalDescribe = Schema.getGlobalDescribe();
            
            // A list to put our sObjects in so we can bulk update them
            List<SObject> sobjList = new List<SObject>();
            
            // Iterate through the sObjectMap param
            for(Sobject obj : sobjectMap.values()) {
                String parentId = String.valueOf(obj.get('ParentId'));
                
                // Iterate through the sObject types and find our parent sObject type by keyPrefix
                for(schema.Sobjecttype objToken : globalDescribe.values()) {
                    DescribeSObjectResult objDef = objToken.getDescribe();
                    
                    if(null != objDef.keyPrefix && parentId.startsWith(objDef.keyPrefix)) {
                        // We found our sObject type now check if it has the magic field
                        if(objDef.fields.getMap().containsKey(LAST_CHAT_DATE_FIELD_NAME)) {
                            // This sObject type has the magic field so create a new sObject with the 
                            // parentId and set the last chatter date to the crated date of the
                            // the FeedItem or FeedComment)
                            Sobject targetObj = objToken.newSObject(parentId);
                            targetObj.put(LAST_CHAT_DATE_FIELD_NAME, (Datetime) obj.get('CreatedDate'));
                            sobjList.add(targetObj);
                        }
                        
                        // We found our sObject so break out
                        break;
                    }
                }
            }
            
            if(!sobjList.isEmpty()) {
                // Call the update DML method, try to save what we can and deal with the exceptions
                List<Database.SaveResult> saveResultList = database.update(sobjList, false);
                // Iterate through the Save Results
                for(Database.SaveResult sr:saveResultList) {
                    if(!sr.isSuccess()) {
                        // Validation rules and other unknown things can
                        // cause exceptions so deal with it as you see fit
                    }
                }
            }
        } catch(system.Exception ex) {
            // an unknown exception
        }
    }
}

The Triggers

trigger FeedItemLastChatterDate on FeedItem (after insert) {
    ChatterDate.updateChatterTimestamp(trigger.newMap);
}

trigger FeedCommentLastChatterDate on FeedComment (after insert) {
    ChatterDate.updateChatterTimestamp(trigger.newMap);
}

And no sample is complete without a nice unit test.

@isTest
private class ChatterDateTest {

    static testMethod void singleUnitTest() {
        Account testAccount = new Account(Name='TestAccount');
        insert testAccount;
        
        FeedItem post = new FeedItem();
        post.parentId = testAccount.Id;
        post.body = 'Test chatter post';
        
        FeedComment comment = new FeedComment();
        comment.CommentBody = 'Test chatter comment on test post';
        
        test.startTest();
            insert post;
            comment.FeedItemId = post.Id;
            insert comment;
        test.stopTest();
        
        Account resultAccount = [SELECT LastChatterDate__c FROM Account WHERE Id = :testAccount.Id];
        AccountFeed resultFeed = [Select (Select Id, FeedItemId, ParentId, CreatedDate From FeedComments Where Id = :comment.Id) From AccountFeed Where Id = :post.Id];
                                  
        system.assertEquals(resultAccount.LastChatterDate__c, resultFeed.FeedComments[0].CreatedDate);
    }
    
    static testMethod void multiUnitTest() {
        Account testAccount = new Account(Name='TestAccount');
        insert testAccount;
        
        Contact testContact = new Contact(LastName='TestContact');
        insert testContact;
        
        List<FeedItem> feedItemList = new List<FeedItem>();
        
        FeedItem accountPost = new FeedItem();
        accountPost.parentId = testAccount.Id;
        accountPost.body = 'Test chatter post';
        feedItemList.add(accountPost);
        
        FeedItem contactPost = new FeedItem();
        contactPost.ParentId = testContact.Id;
        contactPost.body = 'Test chatter post';
        feedItemList.add(contactPost);
        
        test.startTest();
            insert feedItemList;
        test.stopTest();
        
        Account resultAccount = [SELECT LastChatterDate__c FROM Account WHERE Id = :testAccount.Id];
        AccountFeed resultAccountFeed = [SELECT CreatedDate FROM AccountFeed WHERE Id = :accountPost.Id];
        system.assertEquals(resultAccountFeed.CreatedDate, resultAccount.LastChatterDate__c);
        
        Contact resultContact = [SELECT LastChatterDate__c FROM Contact WHERE Id = :testContact.Id];
        ContactFeed resultContactFeed = [SELECT CreatedDate FROM ContactFeed WHERE Id = :contactPost.Id];
        system.assertEquals(resultContactFeed.CreatedDate, resultContact.LastChatterDate__c);
    }
}

Here is the Last Chatter Date field definition:

    <fields>
        <fullName>LastChatterDate__c</fullName>
        <description>Used to record the date and time of the last chatter post or comment.</description>
        <externalId>false</externalId>
        <inlineHelpText>The date and time of the last Chatter post or comment.</inlineHelpText>
        <label>Last Chatter Date</label>
        <required>false</required>
        <trackFeedHistory>false</trackFeedHistory>
        <type>DateTime</type>
    </fields>

Discussion

You will need the LastChatterDate__c field on the Account and Contact sObject for the unit test to work.