A trigger that sets the contact owner to whomever most recently created a case on the record. Also sets the account owner to whomever most recently created a case on it

CaseContactOwnerTrigger

/*
 Write a trigger that sets the contact owner to whomever most recently created a case on the record
 In the same trigger set the account owner to whomever most recently created a case on it
 */

//After trigger to access Case createdById
trigger CaseContactOwner on Case (after insert) {
	
    for(Case myCase : Trigger.new) {
        //make sure there's a contact for null pointer exceptions
        if(myCase.ContactId != null){
            Contact myCon = [SELECT Id
                               FROM Contact
                              WHERE Id = :myCase.ContactId];
            
            //Update the contact
            myCon.OwnerId = myCase.CreatedById;
            update myCon;
        }
        
        //Repeat for the account
        if(myCase.AccountId != null) {
            Account myAcc = [SELECT Id
                               FROM Account
                              WHERE Id = :myCase.AccountId];
            
            //update the account
            myAcc.OwnerId = myCase.CreatedById;
            update myAcc;
        }
    }
}

//Test Class

@isTest
private class CaseContactOwnerTest {
	//Only one scenario
    @isTest static void createCaseOnContact(){
        //Create a user
        User myUser 	 = new User();
        myUser.FirstName = 'Matt';
        myUser.LastName  = 'Lacey';
        myUser.Email	 = 'mlacey@trello.com';
        myUser.UserName  = 'mlacey@trello.com';
        myUser.Alias 	 = 'mlacey';
        myUser.ProfileId = [SELECT Id
                             FROM Profile
                            WHERE Name = 'System Administrator'].Id;
        myUser.TimeZoneSidKey 	 = 'America/Denver';
        myUser.LocaleSidKey 	 = 'en_US';
        myUser.EmailEncodingKey  = 'UTF-8';
        myUser.LanguageLocaleKey = 'en_US';
        myUser.ManagerId = null;
        insert myUser;
        
        Contact myCon   = new Contact();
        myCon.FirstName = 'Tessa';
        myCon.LastName  = 'Trimble';
        myCon.OwnerId   = myUser.Id;
        insert myCon;
        
        Case myCase		= new Case();
        myCase.Subject  = 'Windsor Hotels Repair AC Vent';
        myCase.ContactId = myCon.Id;
        insert myCase;
        
        Contact updatedCon = [SELECT OwnerId
                                FROM Contact
                               WHERE Id = :myCon.Id];
        
        System.assertEquals(UserInfo.getUserId(), updatedCon.OwnerId);
    }
    
    @isTest static void createCaseOnAccount(){
        //Create a user
        User myUser 	 = new User();
        myUser.FirstName = 'Matt';
        myUser.LastName  = 'Lacey';
        myUser.Email	 = 'mlacey@trello.com';
        myUser.UserName  = 'mlacey@trello.com';
        myUser.Alias 	 = 'mlacey';
        myUser.ProfileId = [SELECT Id
                             FROM Profile
                            WHERE Name = 'System Administrator'].Id;
        myUser.TimeZoneSidKey 	 = 'America/Denver';
        myUser.LocaleSidKey 	 = 'en_US';
        myUser.EmailEncodingKey  = 'UTF-8';
        myUser.LanguageLocaleKey = 'en_US';
        myUser.ManagerId = null;
        insert myUser;
        
        Account myAcc = new Account();
        myAcc.Name = 'Windsor Hotels';
        myAcc.OwnerId = myUser.Id;
        insert myAcc;
        
        Case myCase = new Case();
        myCase.Subject = 'Repair TV units';
        myCase.AccountId = myAcc.Id;
        insert myCase;  //Case created Id will be me
        
        Account updatedAcc = [SELECT OwnerId
                                FROM Account
                               WHERE Id = :myCase.AccountId];
        
        System.assertEquals(UserInfo.getUserId(), updatedAcc.OwnerId);
    }
}