Deleting child records on a parent

Question

I am trying to write a before update trigger that deletes all of the child records related to a parent when a checkbox is checked. I have another trigger that fires after update to rebuild these items so it has to happen before.

I am not getting an error when I save the code but I am getting an error when I check the box on a record. It says:

Error: Apex trigger DeleteRecipeSheetItems caused an unexpected exception, contact your administrator: DeleteRecipeSheetItems: execution of BeforeUpdate caused by: System.FinalException: Collection is read-only: External entry point"

My code:

trigger DeleteRecipeSheetItems on Recipe_Sheet__c (before update) {

    Set<Id> recShts = Trigger.newmap.keySet();
    List<Recipe_Sheet_Item__c> RecipeItemList = new List<Recipe_Sheet_Item__c>();


    for(Recipe_Sheet__c rec :trigger.new){
        if(rec.Recalculate_Recipe_Sheet_Items__c == True){
            recShts.add(rec.Id); //Stores the Id of the Recipe_Sheet__c in the list
        }
    }

    RecipeItemList = [SELECT Id,Recipe_Sheet__c,Item__c from Recipe_Sheet_Item__c 
        where Recipe_Sheet__c IN: recShts];        

    if(RecipeItemList.size() > 0)
        delete RecipeItemList;
}

Answer 1

Building on Eric's answer, the code can be cut down to this:

trigger DeleteRecipeSheetItems on Recipe_Sheet__c (before update) {
    Set<Id> ids = new Set<Id>();
    for(Recipe_Sheet__c r : Trigger.new) {
        if (r.Recalculate_Recipe_Sheet_Items__c) {
            ids.add(r.Id);
        }
    }
    delete [SELECT Id from Recipe_Sheet_Item__c where Recipe_Sheet__c IN :ids];        
}

Answer 2

Are you getting the error on this line:

recShts.add(rec.Id);

If so, it is probably because you are setting it equal to the newMap.keyset.

Not sure why you are initializing it with the newMap.keyset().

Just change the line to Set<ID recShts = New Set<ID>()

since you are populating it later

Answer 3

You cannot delete same record in its before update Trigger. Your trigger is defined on "Recipe_Sheet__c ", You are collecting its ID and trying to delete same record. You can delete other records of same Object, but not record itself.