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Here is the latest up to date working Component,

Controller, and my Unit Test (tricky) for

the GenericHistoryComponent

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The bug fixes here are: (see previous posts for specifics.)

1. ParentID vs ObjectNameID  (fixed so this works for standard objects.)

2. OldValue was supposed to read NewValue.

3. If no object passed in, return empty list.

4. Opportinuty history object has the suffix FieldHistory.

5. v24 of the Salesforce API test classes cannot see existing org data by default.  Added (SeeAllData=true) to the test class.

6. Fixed the display of the history record created date to show as the users local time instead of GMT.

Component:

<apex:component controller="GenericHistoryComponentController">
    <!-- Attribute Definition -->
    <apex:attribute name="myObject" description="Object we wish to view the history of" type="SObject" required="true" assignTo="{!myObject}" />
    <apex:attribute name="recordLimit" description="Number of lines of history to display" type="Integer" required="false" assignTo="{!recordLimit}" />
    
    <!-- Object History Related List -->
    <apex:pageBlock title="{!objectLabel} History">
        <apex:pageBlockTable value="{!ObjectHistory}" var="History" >
            <apex:column headerValue="Date"  value="{!History.thedate}"/>
            <apex:column headerValue="User">
                <apex:outputLink value="/{!History.userId}"> {!History.who} </apex:outputLink>
            </apex:column>
            <apex:column headerValue="Action"><apex:outputText escape="false" value="{!History.action}"/></apex:column>
        </apex:pageBlockTable>
    </apex:pageBlock>
</apex:component>

Controller:

public class GenericHistoryComponentController {
    
    // External variables
    public SObject myObject {get; set;}
    public Integer recordLimit {get; set;}
    public static String objectLabel {get;}
    
    // Internal Variables
    public objectHistoryLine[] objectHistory; 

    public static final Map<String, Schema.SObjectType> mySObjectTypeMap = Schema.getGlobalDescribe();
    public static Map<String, Schema.SObjectField> myObjectFieldMap;
    public static List<Schema.PicklistEntry> historyFieldPicklistValues;
    
    public List<objectHistoryLine> getObjectHistory(){
	// if no object passed in, return empty list
        if (myObject == null) {
            return new List<objectHistoryLine>();
        }

        Id myObjectId = String.valueOf(myObject.get('Id'));
        Schema.DescribeSObjectResult objectDescription = myObject.getSObjectType().getDescribe();

        myObjectFieldMap = objectDescription.fields.getMap();
        objectLabel = String.valueOf(objectDescription.getLabel());
        
        //Get the name of the history table
        String objectHistoryTableName = objectDescription.getName();
        
        //ID field name         
		string ObjectIdName;
		
		//if we have a custom object we need to drop the 'c' off the end before adding 'History' to get the history tables name        
		if (objectDescription.isCustom()){            
			objectHistoryTableName = objectHistoryTableName.substring(0, objectHistoryTableName.length()-1);
			ObjectIdName = 'ParentId';        
		}        
		else{
		    ObjectIdName = objectHistoryTableName+ 'Id';
		}
		
	if(objectHistoryTableName == 'Opportunity') {objectHistoryTableName = objectHistoryTableName + 'FieldHistory';}
	else {objectHistoryTableName = objectHistoryTableName + 'History';}
        
        Schema.DescribeFieldResult objectHistoryFieldField = mySObjectTypeMap.get(objectHistoryTableName).getDescribe().fields.getMap().get('Field').getDescribe();
        historyFieldPicklistValues = objectHistoryFieldField.getPickListValues();
                    
        list<objectHistoryLine> objectHistory = new list<objectHistoryLine>();
        
        String prevDate = '';
                
        if (recordLimit== null){
            recordLimit = 100;
        }
                
        list<sObject> historyList = Database.query( 'SELECT CreatedDate,'+
                                                    'CreatedById,'+
                                                    'Field,'+
                                                    'NewValue,'+
                                                    'OldValue ' +
                                                    'FROM ' + objectHistoryTableName + ' ' +
                                                    'WHERE ' + ObjectIdName + ' =\'' + myObjectId + '\' ' +
                                                    'ORDER BY CreatedDate DESC '+
                                                    'LIMIT ' + String.valueOf(recordLimit));
        
        for(Integer i = 0; i < historyList.size(); i++){
            sObject historyLine = historyList.get(i);
            if ((historyLine.get('newValue') == null && historyLine.get('oldValue') == null) 
                    || (historyLine.get('newValue') != null && !(string.valueOf(historyLine.get('newValue')).startsWith('005') || string.valueOf(historyLine.get('newValue')).startsWith('00G')))
                    || (historyLine.get('oldValue') != null && !(string.valueOf(historyLine.get('oldValue')).startsWith('005') || string.valueOf(historyLine.get('oldValue')).startsWith('00G')))){
                objectHistoryLine tempHistory = new objectHistoryLine();
                // Set the Date and who performed the action
                if (String.valueOf(historyLine.get('CreatedDate')) != prevDate){
                    tempHistory.theDate = datetime.valueof(historyLine.get('CreatedDate')).format();
                    tempHistory.userId = String.valueOf(historyLine.get('CreatedById'));
                    tempHistory.who = String.valueOf(historyLine.get('CreatedById'));
                }
                else{
                    tempHistory.theDate = '';
                    tempHistory.who = '';
                    tempHistory.userId = String.valueOf(historyLine.get('CreatedById'));
                }
                prevDate = String.valueOf(historyLine.get('CreatedDate'));
                
                // Get the field label
                String fieldLabel = GenericHistoryComponentController.returnFieldLabel(String.valueOf(historyLine.get('Field')));
                
                // Set the Action value
                   if (String.valueOf(historyLine.get('Field')) == 'created') {    // on Creation
                       tempHistory.action = 'Created.';
                  }
                  else if (historyLine.get('oldValue') != null && historyLine.get('newValue') == null){ // when deleting a value from a field
                      // Format the Date and if there's an error, catch it and re
                      try {
                         tempHistory.action = 'Deleted ' + Date.valueOf(historyLine.get('oldValue')).format() + ' in <b>' + fieldLabel + '</b>.';
                    } catch (Exception e){
                         tempHistory.action = 'Deleted ' + String.valueOf(historyLine.get('oldValue')) + ' in <b>' + fieldLabel + '</b>.';
                    }
                  }
                  else{  // all other scenarios
                    String fromText = '';
                    if (historyLine.get('oldValue') != null) {
                         try {
                              fromText = ' from ' + Date.valueOf(historyLine.get('oldValue')).format();
                         } catch (Exception e) {
                              fromText = ' from ' + String.valueOf(historyLine.get('oldValue'));
                         }
                    }
                    
                    String toText = '';
                    if (historyLine.get('newValue') != null) {
                        try {
                             toText = Date.valueOf(historyLine.get('newValue')).format();
                        } catch (Exception e) {
                             toText = String.valueOf(historyLine.get('newValue'));
                        }
                    }
                    if (toText != ''){
                        tempHistory.action = 'Changed <b>' + fieldLabel + '</b>' + fromText + ' to <b>' + toText + '</b>.';
                    }
                    else {
                        tempHistory.action = 'Changed <b>' + fieldLabel;
                    }
                   }
                   
                   // Add to the list
                   objectHistory.add(tempHistory);
            }
         }
         
         List<Id> userIdList = new List<Id>();
         for (objectHistoryLine myHistory : objectHistory){
             userIdList.add(myHistory.userId);
         }
         Map<Id, User> userIdMap = new Map<ID, User>([SELECT Name FROM User WHERE Id IN : userIdList]);
         
         for (objectHistoryLine myHistory : objectHistory){
             if (userIdMap.containsKey(myHistory.userId) & (myHistory.who != '') ){
                 myHistory.who = userIdMap.get(myHistory.who).Name;
             }
         }
         
         return objectHistory;
    }    
    
    // Function to return Field Label of a object field given a Field API name
    public Static String returnFieldLabel(String fieldName){

        if (GenericHistoryComponentController.myObjectFieldMap.containsKey(fieldName)){
            return GenericHistoryComponentController.myObjectFieldMap.get(fieldName).getDescribe().getLabel();
        }
        else {
            for(Schema.PicklistEntry pickList : historyFieldPicklistValues){
                if (pickList.getValue() == fieldName){
                    if (pickList.getLabel() != null){
                        return pickList.getLabel();
                    }
                    else {
                        return pickList.getValue();
                    }
                }
            }
        }
        return '';
    }
    
    // Inner Class to store the detail of the object history lines    
    public class objectHistoryLine {

        public String theDate {get; set;}
        public String who {get; set;}
        public Id userId {get; set;} 
        public String action {get; set;}
    }
}

Unit Test:

@isTest(SeeAllData=true)
public class GenericHistoryControllerTester {

  static testMethod void myTestGenericHistoryComponentController() {
        
        //Unit test by: Mike Katulka @ www.itedgecrm.com - mike@itedgecrm.com
        //Component by: http://boards.developerforce.com/t5/Visualforce-Development/History-related-List-using-Visualforce/td-p/151785
        //83% coverage
        
        //***Instructions MUST READ***
        //We cannot generate History from a unit test, and unable to manually insert records into the History tables from code.  This seems to be a limitation of Salesforce unit testing.
        //Created for Winter 12 org testing, updated to work with v24 API (SeeAllData=true)
        //Steps:
        // 1. Enable History Tracking for Account.
        // 2. You will need to declaratively make changes to an existing Account.  
        // 3. Create history in Sandbox and Production.
        // 4. Then run this test/deploy.
        // This test will look at the existing history in your org, and operate off of it.  
        
        //create controller
		GenericHistoryComponentController controller = new GenericHistoryComponentController();
		//null myObject
		system.assert(controller.getObjectHistory().size() == 0);

        //Account allows for history in related lists in visualforce, this is simply a standard object being used to get code coverage so you can use the component for other custom and standard objects.
        system.assert([select Accountid from accounthistory where isdeleted=false limit 1].size() == 1); //limit in case of 50,000 record governor limit
        string accid = [select Accountid from accounthistory where isdeleted=false limit 1].accountid;
        
		//Set myObject
		controller.myObject = [select id from Account where id=:accid];				
        system.assert(controller.getObjectHistory().size() > 0);        
  }

}

Mike Katulka

www.itedgecrm.com