Hi,

I have a trigger in Sandbox that updates a checkbox field to true if an attachment is attached to an expense record.  I have also created a test class with a code coverage of 75%.

Trigger:
 

trigger ExpenseAttachment on ContentDocumentLink (after insert) {
String tempParentId;
	Set<Id> setParentId = new Set<Id>();
	List<Expense__c> Expenselst = new List<Expense__c>();
	
 for (ContentDocumentLink cdl : trigger.new ) {
			tempParentId = cdl.LinkedEntityId;
	 
			if (tempParentId.left(3) =='a0X') {
				System.debug('Debug : found a0X');
				System.debug('Debug : content document id ' + cdl.ContentDocumentId );
				setParentId.add(cdl.LinkedEntityId);
			}
		}
	Expenselst = [select Id , HasAttachment__c from Expense__c where Id IN :setParentId];
	 
	 For(Expense__c e : Expenselst)
	 {
		e.HasAttachment__c = True;
	 }

	 update Expenselst;
}

Test Class:

@istest
private class TestExpenseAttachment {
    @isTest static void TestExpensewithAttachment(){
        //Test data setup
        //Create an Expense
        
        Expense__c Exp = new Expense__c();
        insert Exp;
        
        //Create a ContentVersion
    
        ContentVersion ContentDoc = new ContentVersion();
        ContentDoc.Title = 'My Doc';
        ContentDoc.ContentUrl= 'test.com';
        Insert ContentDoc;
        
		ContentVersion testContent = [SELECT id, ContentDocumentId FROM ContentVersion where Id = :ContentDoc.Id];
        
        // Create a ContentDocumentLink
        ContentDocumentLink ContentDL = new ContentDocumentLink();
        ContentDL.ContentDocumentId = testcontent.contentdocumentid;
        ContentDL.LinkedEntityId=Exp.id;
        ContentDL.ShareType='I';
        Insert ContentDL;
        
        //Assert checkbox field
        System.assert(Exp.HasAttachment__c=TRUE);
    }
}

When I try to deploy this to production through change sets, I get the following error:

"Your organization's code coverage is 8%. You need at least 75% coverage to complete this deployment. Also, the following triggers have 0% code coverage. Each trigger must have at least 1% code coverage.
    ExpenseAttachment"

This is ny first time deploying a trigger.  Why is the code coverage of this trigger 75% in sandbox but 0% in production and how can I increase the code coverage?  Also how can I increase the org's code coverage?

Thanks.