List has more than 1 row for assignment to SObject: External entry point

Question

I know that there's already several posts on SE for this error but none of them explain this for me.

I'm testing my code by editing one Project Deadline date field, the upProjects size is one and as you can see, I'm adding the Project Month records to a list (existPMs).

But I still get the error

List has more than 1 row for assignment to SObject: External entry point

when I execute the code.

trigger ProjectLifecycle on MPM4_BASE__Milestone1_Project__c (after insert, after update) {

    List<WEID__c> weids = WEID__c.getall().values();
    Set<Id> validRecordTypeIds = new Set<Id>();

    // add Project Record Type Ids from Custom Setting to list of valid Ids      
    for(WEID__c weid: weids) {
        try {
            validRecordTypeIds.add(weid.WEProjectId__c);
        } catch (System.StringException e) {
            System.debug(
                System.LoggingLevel.ERROR,
                'Invalid Record Type Id ' + weid.WEProjectId__c
            );
        }
    }

    List<MPM4_BASE__Milestone1_Project__c> insProjects = new List<MPM4_BASE__Milestone1_Project__c>();
    List<MPM4_BASE__Milestone1_Project__c> upProjects = new List<MPM4_BASE__Milestone1_Project__c>();
    Map<Id,Integer> projectDuration = new Map<Id,Integer>();
    Map<Integer,String> monthName = new Map<Integer,String>{1 => 'January', 2 => 'February', 3 => 'March', 4 => 'April', 5 => 'May', 6 => 'June', 7 => 'July', 8 => 'August', 9 => 'September', 10 => 'October', 11 => 'November', 12 => 'December'};
    List<Project_Month__c> newPMs = new List<Project_Month__c>();
    List<Project_Month__c> existPMs = new List<Project_Month__c>();

    for(MPM4_BASE__Milestone1_Project__c p : Trigger.New){

        //check that Project record type is valid
        if(validRecordTypeIds.contains(p.RecordTypeId))
        {
            try{
                MPM4_BASE__Milestone1_Project__c oldP = Trigger.OldMap.get(p.Id);

                if(oldP.MPM4_BASE__Kickoff__c != p.MPM4_BASE__Kickoff__c
                   ||
                   oldP.MPM4_BASE__Deadline__c != p.MPM4_BASE__Deadline__c)
                {
                    upProjects.add(p);
                    projectDuration.put(p.Id, p.MPM4_BASE__Kickoff__c.monthsBetween(p.MPM4_BASE__Deadline__c));
                }
            //if old.Map is null, record has been inserted & project months should be added
            }catch(system.NullPointerException e){
                insProjects.add(p);
                projectDuration.put(p.Id, p.MPM4_BASE__Kickoff__c.monthsBetween(p.MPM4_BASE__Deadline__c));
            }
        }
    }

    if(insProjects.size() > 0){

        for(MPM4_BASE__Milestone1_Project__c p : insProjects){

            Date k = p.MPM4_BASE__Kickoff__c;
            Integer km = p.MPM4_BASE__Kickoff__c.month();
            Integer dur = projectDuration.get(p.Id);

            //create a set of Project Month records for each Project
            for(Integer m = km; m < km + dur + 1; m++){

                Project_Month__c pm = new Project_Month__c();
                pm.Project__c = p.Id;
                pm.Name = monthName.get(m)+' '+ k.addMonths(m - 1).year();
                pm.month__c = k.addMonths(m - km + 1).toStartOfMonth() - 1;
                pm.Project_Value__c = p.Opportunity_Total_Net_FSR__c;
                newPMs.add(pm);
                system.debug('newPMs contains ' + newPMs);
            }
        }
    }

    system.debug('upProjects size is ' + upProjects.size() );

    if(upProjects.size() > 0){

        for(MPM4_BASE__Milestone1_Project__c p : [SELECT Id,
                                                 (SELECT Id FROM Project_Months__r) FROM MPM4_BASE__Milestone1_Project__c
                                                   WHERE Id IN :upProjects]){
                                                       existPMs.add(p.Project_Months__r);
                                                   }
        delete existPMs;

        for(MPM4_BASE__Milestone1_Project__c p : upProjects){

            Date k = p.MPM4_BASE__Kickoff__c;
            Integer km = p.MPM4_BASE__Kickoff__c.month();
            Integer dur = projectDuration.get(p.Id);

            for(Integer m = km; m < km + dur + 1; m++){

                Project_Month__c pm = new Project_Month__c();
                pm.Project__c = p.Id;
                pm.Name = monthName.get(m)+' '+ k.addMonths(m - 1).year();
                pm.month__c = k.addMonths(m - km + 1).toStartOfMonth() - 1;
                pm.Project_Value__c = p.Opportunity_Total_Net_FSR__c;
                newPMs.add(pm);
                system.debug('newPMs contains ' + newPMs);
            }
        } 
    }
    insert newPMs;
}

I'd used some code from Trailhead as the template for this! (The last sample code on the page).

I've also tried changing the above to

List<MPM4_BASE__Milestone1_Project__c> pList = [SELECT Id,
                                               (SELECT Id FROM Project_Months__r) FROM MPM4_BASE__Milestone1_Project__c
                                                 WHERE Id IN :upProjects];{
                                               existPMs.add(pList.Project_Months__r);
                                           }
delete existPMs;

but that causes the error

Initial term of field expression must be a concrete SObject:

Answer 1

Fix this:

existPMs.add(p.Project_Months__r)

needs to be

existPMs.addAll(p.Project_Months__r)

The run in dev console and see where the error is coming from. It may not even be in the trigger at all but may be elsewhere.

Answer 2

I'm just looking at the last updated code and it seems to be incorrect:-

Existing code:-

List<MPM4_BASE__Milestone1_Project__c> pList = [SELECT Id,
                                               (SELECT Id FROM Project_Months__r) FROM MPM4_BASE__Milestone1_Project__c
                                                 WHERE Id IN :upProjects];{
                                               existPMs.add(pList.Project_Months__r);
                                           }

delete existPMs;

Issue:- you are using list value as a instance of MPM4_BASE__Milestone1_Project__c, you would need to use list methods/ mechanism to access list members for e.g. pList[0].Project_Months__r

Suggested code:-

For(MPM4_BASE__Milestone1_Project__c project = [SELECT Id,
                                               (SELECT Id FROM Project_Months__r) FROM MPM4_BASE__Milestone1_Project__c
                                                 WHERE Id IN :upProjects])
{
   existPMs.add(project .Project_Months__r);
}
delete existPMs;