"Invalid bind expression type of Schema.SObjectField" when attempting APEX Trigger

Question

Here's my trigger:

 trigger updateFieldUponEmail on EmailMessage (before insert) {
        //find parentId of EmailMessage
        //where case.Id = parentId, increment number field isUpdated
        SObjectField EmailMessageID = EmailMessage.parentID;
        for(Case c: [SELECT timesUpdated__c FROM Case WHERE Id = :EmailMessageID]){
            c.timesUpdated__c++;
        }
    }

I want it to increment a number field of a case that the EmailMessage is a child of. I'm fairly new to Java/APEX so I'm not sure how exactly to do this, but I got this far. However, I keep getting the error: Invalid bind expression type of Schema.SObjectField. What can I do? Thank you!

Answer 1

You cannot bind (:variable name) an sObjectField in an soql query. It has to be an actual variable so simply change the variable type to the ID type which will hold the value instead of sObjectField which is the entire field object:

trigger updateFieldUponEmail on EmailMessage (before insert) {
        Set<ID> caseIDs = new Set<ID>();

        for(EmailMessage msg : trigger.new){
            if(msg.parentID.getsObjectType() == Case.sObjectType)
               caseIDs.add(msp.parentID);
        }

        Case[] casesToUpdate = [SELECT timesUpdated__c FROM Case WHERE ID IN :CaseIDs];
        for(Case c: casesToUpdate){
            c.timesUpdated__c = c.timesUpdated__c == null ? 1 : c.timesUpdated__c +1;
        }

     update casesToUpdate;

    }

I have updated the code to bulkify it