Trigger to set a look up field to blank based on criteria change in a picklist

Question

I am having a problem creating a trigger that sets a lookup field on the child object to blank when a picklist value changes on the child object. Background:

There are two Objects related by a lookup relationship Building__c (Child) and Warehouse__c (Parent)

The Child Object Building__c has three fields, Name, Warehouse_Name__c (Lookup to Warehouse), and Status(picklist). I need a trigger that when Status is changed to Complete the field Warehouse_Name__c should be blank without deleting the record.

trigger Diss on Building__c (after insert, after update) { 
    Map<ID, Warehouse__c> parentware = new Map<ID, Warehouse__c>(); 
    List<Id> listIds = new List<Id>();

    for (Building__c childObj : Trigger.new) {
        if(childObj.Status__c != null && childObj.Status__c.equals('Complete')){
            //System.debug("ids for query: " + childObj.Id);
            listIds.add(childObj.Id);
        }
    }

    parentware = new Map<Id, Warehouse__c>([SELECT id, Name,(SELECT ID, Status__c, Warehouse_Name__c FROM Building__r Where Status__c = 'Complete') FROM Warehouse__c WHERE ID IN :listIds]);

    // for (Warehouse__c build: Trigger.new){
    //  System.debug("2nd loop: " + build.toString());
    //    Warehouse__c myParentOpp = parentware.get(build.Warehouse_Name__c);
    //    myParentOpp.Name = build.Warehouse_Name__c;
    //    myParentOpp.Name = null;
    // }

    List<Building__c> buildUpdate = new List<Building__c>();

    for (Building__c bui : Trigger.new) {
        Warehouse__c wl = parentware.get(bui.Warehouse_Name__c);
        if (bui.Warehouse_Name__c.equals(wl.Name)) {
            bui.Warehouse_Name__c = null;
            buildUpdate.add(bui);
        }
    }

    update buildUpdate;
}

Answer 1

You've got way too much code. Consider taking some of the basic APEX webinars on Triggers..

To do what you described, you could do just this (note that I made it a before trigger instead of an after trigger):

trigger Diss on Building__c (before insert, before update) { 
    for (Building__c childObj : Trigger.new) {
        if(childObj.Status__c != null && childObj.Status__c.equals('Complete')){
            childObj.Warehouse_Name__c = null;
        }
    }
}