[SalesForce] Updating fields from multiple objects on a single visualforce page

There are two standard controllers, one gives you access to a single sObject, the other gives you a list.

When your page only used the attribute standardController, you had the single-object controller in your page. This requires you to pass in an ID value if you want to see a record, it queries for you, and you then have access to a single sobject instance by the exact API name of that object.

<apex:page standardController="My_Object__c">

I can now bind to an instance (you could also say a record) from that object by name, so the following is allowed:

<apex:inputField value="{!My_Object__c.name}"/>

But if you want the controller that is a list of data, that one is actually called "Standard List Controller" or StandardSetController, depending on the docs. The way you make your page use that one is using the recordSetVar attribute on your page.

When you do that, you transmogrify the way the controller works, and you no longer have access to a single record any more, and the list will be populated based on the last used List View by that user.

<apex:page standardController="My_Object__c" recordSetVar="mylist">

  <apex:outputField value="{!My_Object__c.name}"/> <--this is not allowed

  <apex:dataList value="{!mylist}" var="item"> <-- this is allowed
    <apex:outputField value="{!item.Name} <-- so is this
  </apes:dataList>

So it's an either or scenario. Either your page uses standardController and you have access to the one record, or it uses standardController + recordSetVar and you have access to the list of records.

But given you already are using an extension, if you need a list of Opportunity_Package__c records in your page, rather than trying to use recordSetVar like you're doing, I would suggest you just create a new property in your extension and bind to that.

public List<Opportunity_Package__c> listOfOppPacks {get;set;}

Then you can just populate it yourself when you need to.

The code that you initially provided is going about things the hard way.

My preferred method of taking care of this would be to make ObjectB__c into a separate variable in your controller, and then use that in the visualforce binding.

public class UploadControllerVendorProdRev {

    // This member variable, thedateinput, won't be needed
    // public Date thedateinput {get;set;}

    private objectA__c parent {get;set;}

    // Declare an ObjectB__c variable to be used later
    public objectB__c objBVar{get;set}

    public UploadControllerVendorProdRev(ApexPages.StandardController controller) { 
        this.parent = (ObjectA__c)controller.getRecord();

        // Here, in your controller's constructor, we need to initialize
        //  objBVar. If it isn't initialized here, you'd probably get a 
        //  NullPointerException when you attempt to reference it in your page

        // We can also set the value of Parent__c in the constructor for ObjectB__c
        objBVar = new ObjectB__c(Parent__c = this.parent.Id);
    }
}

Because we're taking care of the initialization in the controller's constructor, you don't need to do it in your saveCustomAttachment() method any more. In fact, if you leave that initialization code in your saveCustomAttachment() method, you'll likely run into issues (a variable redeclaration error, or perhaps the date being overwritten).

private Database.SaveResult saveCustomAttachment() {

    // This initialization is taken care of in the controller's constructor
    //objBVar = new ObjectB__c();

    // This line was moved into the call to the constructor for ObjectB__c
    //   (as a parameter).
    // This line is no longer needed either
    //objBVar.Parent __c = parent.Id; 

    // We can get rid of this line by changing your Visualforce to use
    //   {!objBVar.Document_Date__c}
    // Trying to use 'thedateinput' would cause any Date stored in 
    //   objBVar.Document_Date__c to be overwritten.
    //objBVar.Document_Date__c = thedateinput ;

    // In the end, this should be the only remaining line in this method
    return Database.insert(obj);    
}

To tie this all together, we need to update the binding that you're using in your Visualforce page

<apex:inputfield id="someId" value="{!objBVar.Document_Date__c}" required="false"/>

The way this works is that Salesforce will first attempt to find an objBVar in the standard controller. When it doesn't find such a variable in the standard controller, it looks for the variable in your custom controller (or controller extension).

Since we declare objBVar as public, Salesforce has no problem finding it. The input field is then bound to the Document_Date__c field of your objBVar object, which will have the value from your input field after you press save (and not a moment before that!).