Comparing Two Lists of Related Objects

Question

I have two custom objects, Question__c and Response__c. There is a lookup field named Related_Question__c on the Response__c object to relate the two. In my VF controller I have two Lists, one for each object. My objective is to find all records in the Question List that do not have a match in the Response List (ie questions that haven't been answered yet).

I believe I need to use a map for this but am unsure of exactly how to accomplish this. Any advice would be appreciated. Thanks in advance.

Here's what I have so far to build the two Lists:

List <Question__c> questions = [select Id, Name From Question__c];
List <Response__c> questionResponses = [select Id, Name, Related_Question__c From Response__c Where Related_Question__c IN: questions};

Answer 1

Can you try something like this ?

List<Id> l = new List<Id>();
Map<Id,Response__c> qr = new Map<Id,Response__c>();
for(Response__c r : questionResponses)
{

    qr.put(r.Related_Question__r.Id,r);
}
for(Question__c q :  questions)
{

  if(qr.get(q.Id) == null)
 {
      l.add(q.Id);    
}

}
System.debug('~~~ Non matching Ids '+ l);

Answer 2

Try having one SOQL and check if the list of response for that question is empty like below (*Please add a where clause or limit your query)

List<questions__c> questions = [select id,Name,(select Id, Name, Related_Question__c From Responses__r) from question__c];
    for (question__c que: questions) {
        if (que.responses__r.isempty()) {
            //Response is missing for this question.
        } else {
            //From the list of responses for every question in your list do further updates as required.
            for (reponse__c response: que.reponses__r) {
                //do whatever with the response.      
            }
        }
    }  

Answer 3

This will return Questions__c that have no associated with Responses:

   List<Question__c> questionsNoResponse = [select Id, Name from Questions__c where Id not in (select Related_Questions__c from Response__c)]

This will return Questions__c with a list of associated responses. The list size for the associated responses will be 0 if there are no records:

  List<Question__c> questionsAndResponse = [select Id, Name from Questions__c, (select Id, Name from Questions__r) from Questions__c)]

The first SOQL will give you the list you are looking for without writing apex to loop through your original lists. If you use the 2nd query then you can loop like this:

  List<Question__c> quesWithNoResponse = new List<Questions__c>();
  List<Question__c> qList = List<Question__c> questionsAndResponse = [select Id, Name from Questions__c, (select Id, Name from Questions__r) from Questions__c)];

  for (Question__c item: qList){
     if (item.Questions__r.size() = 0){
        quesWithNoResponse.add(item);
     }
  }

NOTE: in the examples above I have assumed that your child relationship API name is Questions__r. You should check the configuration page for the child relationship name for the field Related_Question__c.