I am trying to use a trigger to create a series of cloned records in a custom object, Item.

The Item is a in a Master-Detail relationship with both Opportunity and Event/Issue (aka Issue_Year__c), and I want to change the Event/Issue field for each new, cloned Item. The idea is to split the revenue from one Item across monthly Event/Issue records (for online sales, for example).

However, having tried various cominations of before/after insert and .clone(false/true) function, I can't seem to get it to work.

Here is my code, as it stands:

trigger SeriesItems on Item__c (after insert) {

Set<Id> itemIds = new Set<Id>();

for (integer i = 0; i < Trigger.new.size(); i++){
    if (Trigger.new[i].Item_Sold__c == 'CIPI Membership' && Trigger.new[i].Cloned__c == FALSE){
        itemIds.add(Trigger.new[i].Id);
    }
}

Item__c[] itemList =
    [SELECT Id, Subscription_Term_Months__c, Event_Issue__r.Product__c FROM Item__c WHERE Id IN :itemIds];

if (itemList.size() > 0){
    for (Item__c item :ItemList){
        for (integer n = 1; n < item.Subscription_Term_Months__c; n++){
            Item__c copy = item.clone();
            copy.Cloned__c = TRUE;
            Issue_Year__c ei = 
                [SELECT Id FROM Issue_Year__c WHERE Product__c = :item.Event_Issue__r.Product__c AND CALENDAR_MONTH(Date__c) = 7 LIMIT 1];
            copy.Event_Issue__c = ei.Id;
            insert copy;
        }
    }
}

}

Here is the current error message I get, when using a test class:

System.DmlException: Insert failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, SeriesItems: execution of AfterInsert caused by: System.DmlException: Insert failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, ItemTrigger: execution of BeforeInsert caused by: System.QueryException: List has no rows for assignment to SObject Trigger.ItemTrigger: line 13, column 1: [] Trigger.SeriesItems: line 22, column 1:

And here is the relevant lines of the trigger to which this error refers:

trigger ItemTrigger on Item__c (before insert,after update,after insert) {

    System.debug('Item Trigger');
    Item__c[] item = Trigger.New;
    Item__c[] itemOld = Trigger.Old;
    Double lineNumber;
    String UniqueId='';
    list<ContactAttendance__c> caList = New list<ContactAttendance__c>(); 
        
    for(Integer i = 0; i< item.Size();i++){
        
        if(Trigger.IsInsert && Trigger.IsBefore){
            lineNumber = [select Max_Line_Number__c from opportunity where id = :item[i].Opportunity__c limit 1].Max_Line_Number__c;