Populate address field upon Contact lookup selection

Question

I have about 2 weeks experience in Visualforce and Apex and I am having trouble with displaying an address field related to a contact object. I have a custom Order object called WIMS_Order_c. With a lookup field called Customer_c which is a lookup for the Contact object. All I want to do is upon Customer (contact) lookup selection is to populate the mailing street address label field. However I get the following error after selection:

System.QueryException: List has no rows for assignment to SObject Error is in expression '{!ContactPopulated}' in page orderentrytest2

Class.SFOEController.ContactPopulated: line 13, column 1

Please help. Very much appreciate it. Extension controller and VF code below.

Visual Force

<apex:page standardController="WIMS_Order__c"  extensions="SFOEController" sidebar="false" showHeader="false">
   <apex:sectionHeader title="Order Entry" subtitle="{!WIMS_Order__c.Customer__c}"/>
   <apex:form >
      <apex:pageBlock title="Order Entry">
         <apex:actionRegion >
            <apex:pageBlockSection title="Order Header" columns="2">
               <apex:pageBlockSectionItem >
                  <apex:outputLabel value="Customer"/>
                  <apex:outputPanel >
                     <apex:inputField value="{!WIMS_Order__c.Customer__c}">
                        <apex:actionSupport event="onchange" 
                           action="{!ContactPopulated}"
                           rerender="thePageBlock"
                           status="status"/>
                     </apex:inputField>
                     <apex:actionStatus startText="applying address..." 
                        id="status"/>
                  </apex:outputPanel>
               </apex:pageBlockSectionItem>
            </apex:pageBlockSection>
         </apex:actionRegion>
         <apex:pageBlockSection title="Address" columns="2" id="thePageBlock" rendered="true">
            <apex:pageBlockSectionItem >
               <apex:outputLabel value="Mailing Address"/>
               <apex:outPutField value="{!WIMS_Order__c.Customer__r.MailingStreet}"/>
            </apex:pageBlockSectionItem>
         </apex:pageBlockSection>
      </apex:pageBlock>
   </apex:form>
</apex:page>

EXT Controller

public with sharing class SFOEController 
{
 private ApexPages.StandardController stdCtrl;

 public SFOEController(ApexPages.StandardController std)
 {
  stdCtrl=std;
 }

 public void ContactPopulated()
 {
  WIMS_Order__c cont=(WIMS_Order__c) stdCtrl.getRecord();
  cont.Customer__r =[select MailingStreet, MailingCity from contact where id=:cont.id];
 }
}

Answer 1

Further to Adrian Larson's answer

public with sharing class SFOEController {
  private ApexPages.StandardController stdCtrl;
  public Contact wimsContact {get;set;}

public SFOEController(ApexPages.StandardController std) {
  stdCtrl=std;
}

public void ContactPopulated(){
WIMS_Order__c cont=(WIMS_Order_c) stdCtrl.getRecord();
wimsContact =[select id, MailingStreet, MailingCity from contact where id=:cont.Customer_c]; }

Then change the output field to
<apex:outPutField value="{!wimsContact.MailingStreet}"/>

Let me know how that goes.

Answer 2

The root cause is actually that you are looking for Contact records that have a WIMS_Order__c Id. Your query will always return an empty list because this is an impossible circumstance.

A secondary problem is you are trying to set an SObject as a lookup instead of the SObject's Id. I am fairly certain you cannot use the __r suffix for setting Lookups, so instead you would need to set cont.Customer__c = someId;.