APEX TRIGGER to update a custom field of custom object when a new case record is created

Question

I'm new to Apex and need a help regarding Triggers. I need to update a field(field__c) in custom object (obj1__c) (field of type checkbox) whenever a new record is created in case using email-to-case functionality. Please help me with the trigger Here is the trigger I have written. I didn't get any errors but the code is not working.

trigger CaseAfterInsert on Case (after insert) {
    String contactID;
    set<ID> obj1Id = new set<ID>();
    list<obj1__c> test = new list<obj1__c>();
    for(Case newCases : Trigger.new)
        {
            if(newCases.Subject.equalsIgnorecase('example'))
            {
               test.add(obj1__c);

            }
        }
    test = [Select field__c from obj1__c where Contact__c =: contactID];
    for(obj1__c test1 : test)
    {
    test1.field__c = false;
    System.debug('list test --------------------'+test);
            try {
                   update test1;
                   } 
              catch (Exception e)
              {
              System.debug('Exception occurred--------------'+e);
             }
    }
}

Answer 1

You're using ContactID to query obj1 objects but you are not giving it any value in your trigger. Because of this ContactID is null and probably your query is not returning any records.

Also you are using DML operation inside a for loop which is not recommended. Below code should work.

  trigger CaseAfterInsert on Case (after insert) {
//get new cases into a list
List<Case> newCases = Trigger.new;
//for contact ids to query obj1 records
set<ID> caseIds = new set<ID>();
//to keep obj1 records
list<obj1__c> test = new list<obj1__c>();
List<EmailMessage> = new List<EmailMessage>();
for(Case nc : newCases)
{
    caseIds.add(nc.Id);
}
//parentId is the id of Case related to the email message
messages = [SELECT Id, Subject, ToAddress, ParentId FROM EmailMessage WHERE ParentId IN :caseIds];

if(messages.size() > 0){
    //add ToAddress int a Set and query obj1 according to this set using IN
    for(Integer i = 0; i < messages.size(); i++)        {
        obj1Id.add(messages[i].ToAddress);
    }
    test = [Select field__c from obj1__c where Contact__c IN :obj1Id];
    for(Integer i = 0; i < test.size(); i++){
        test[i].field__c = false;
    }

    System.debug('list test --------------------'+test);
    //update is outside of for loop
    try {
        update test;
    } 
    catch (Exception e)
    {
        System.debug('Exception occurred--------------'+e);
    }

}

}

UPDATE: To address and Subject of email is not on the Case object. It belongs to EmailMessage object. See updated code above.

More info on EmailMessage class; EmailMessage