Hi,

Don't use query inside the for loop because it hits the salesforce governer limit. Always use the collections to avoid query inside for loop.And never perform any operation like sendEmail() inside for loop because there is a limitation of calling SendEmail() method.Never use hardcoded Ids in test class.

I have modified your code and write the test class.

/********************************* Trigger -Start **************************************/

trigger EmailonPost on FeedItem (after insert) {

Set<String> parentIdsSet = new Set<String>();
Set<String> createdUserIdsSet = new Set<String>();
Map<String,List<EntitySubscription>> entitySubMap = new Map<String,List<EntitySubscription>>();
List<Messaging.SingleEmailMessage> email = new List<Messaging.SingleEmailMessage>();
String nspKeyPrefix = Milestone1_Milestone__c.sObjectType.getDescribe().getKeyPrefix();

for (FeedItem f: trigger.new) {
String parentid = f.Parentid;
if( parentid.substring(0,3) == nspKeyPrefix ) {
parentIdsSet.add( f.Parentid );
createdUserIdsSet.add( f.createdById );
}
}
Map<Id,Milestone1_Milestone__c> oppMap = new Map<Id,Milestone1_Milestone__c>([Select Id,Name from Milestone1_Milestone__c where id in :parentIdsSet]);
Map<Id,User> userMap = new Map<Id,User>([Select Id,name from User where id in :createdUserIdsSet]);

for( EntitySubscription es : [select id,ParentId,subscriberid from EntitySubscription where Parentid =:parentIdsSet] )
{
List<EntitySubscription> entityList = new List<EntitySubscription>();
if( entitySubMap.containsKey(es.ParentId) ) {
entityList = entitySubMap.get( es.ParentId );
}
entityList.add( es );
entitySubMap.put( es.ParentId,entityList );
}

for (FeedItem f: trigger.new) {
if( entitySubMap.containsKey( f.Parentid ) ) {
for( EntitySubscription es : entitySubMap.get( f.Parentid )) {
Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
mail.setTargetObjectId(es.subscriberId);
mail.setSaveAsActivity(false);
mail.setSubject('New post on Milestone1_Milestone__c: ' + oppMap.get(f.ParentId).Name);
mail.setPlainTextBody(userMap.get(f.createdById).Name + ' wrote: ' + ' \r\n' + ' \r\n' + f.body);
email.add(mail);
}
}
}

if(email != null ) {
Messaging.sendEmail(email);
}
}

/***************************************** Trigger - End ****************************************/

/**************************************** Test Class - START ****************************************/

@isTest(seeAlldata = true )
private class EmailonPostTester {
static testMethod void unitTest() {
Milestone1_Milestone__c mile = new Milestone1_Milestone__c (
Name = 'test',
/** Mandatory fields of this object.**/
);
insert mile;

Profile p = [select id from profile where Name = :'System Administrator' limit 1];
User pu = new User(profileId = p.id, username = 'tt@ff.com', email = 'pp@cc.com',
emailencodingkey = 'UTF-8', localesidkey = 'en_US',
languagelocalekey = 'en_US', timezonesidkey = 'America/Los_Angeles',
alias='cspu', lastname='lastname');
insert pu;


EntitySubscription es = new EntitySubscription (
ParentId = mile.id,
SubscriberId = pu.Id
);
insert es;

FeedItem feed = new FeedItem (
ParentId = mile.id,
Body = 'Hello'
);
insert feed;
}
}

/****************************************** Test Class - END **********************************/

Please let me know if you have any issue regarding same.

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