Can I add Parent Record Data to a Trigger.NewMap so that I can use dot notation while looping through the map values?

Question

I have a contact trigger that passes Trigger.NewMap and Trigger.OldMap to an apex method, and then in the apex method, the following debug statement returns null for c.Account.Unit_Type__c:

for(Contact c: newMap.values()){
        System.debug('MM ContactTrigger ++++ c.Account.Unit_Type__c = ' + c.Account.Unit_Type__c);
}

Question: I know I can do a separate SOQL query using the map as a WHERE condition to get a list of the contacts and the values of the related Account and Account.Field__c's, and then loop through that list instead of the newMap.values(), but is there any way to ADD that relationship data to the Trigger.NewMap <Id,Contact> map to allow me to traverse the relationships with dot notation while looping through newMap.values like I am trying to do in the code above?

I ask because there is already a lot of code in this class already using the newMap.values() loop but I now want to introduce a new conditional check against related account data. Ideally I'd like to do that without rewriting too much of the class and the easiest way would be if I could just traverse the relationship straight from the map. I am also considering a formula field on Contact but just wanted to check my options first.

Answer 1

First of all, a trigger context won't include any child or parent relationships. These have to be queried for. But assuming you have queried the parent Accounts of the Contacts in the trigger and saved the results in a map

map<id,Account> aIdToAccountMap;

Then, you can modify the sobjects in Trigger.newMap as follows

for (Contact c: Trigger.new)
     c.Account = aIdToAccountMap.get(c.AccountId);

Note that this technique does not work for child relationships; you could not have a map of ContactIds to a list of Attachments as in:

map<ID,Attachment[]> cIdToAttachmentsMap;

and do ...

for (Contact c: Trigger.new)
   c.Attachments = cIdToAttachments.get(c.Id);