Using standard controller's "getRecord" method

There seems to be a common problem using the getRecord function, so I am going to write it down here.  This problem has haunted me a few times now.

Whenever you use a controller extension, extending a standard object, you can use the getRecord() to get the reference to the record.  For example, if you have an extension to the contact object, this can be what your Apex class looks like:

public class ContactExtension
{
    private Contact contact;

    public ContactExtension(ApexPages.StandardController sController)
    {
        this.contact = (Contact)sController.getRecord();
    }
}

Besides the obvious constructor, you will most likely have some other methods inside your class to manipulate the contact.  Let's say you want to provide a form so the user can change email address and phone number of the contact.  Your VisualForce class looks very simple

<apex:page standardController="Contact" extensions="ContactExtension">
    {!greeting}
    <apex:form>
        <apex:inputField value="{!contact.Email" />
        <apex:inputField value="{!contact.MobilePhone}" />
        <apex:commandButton action="{!save}" value="Save" />
    </apex:form>
</apex:page>

The {!greeting} is going to display some greeting messages.  That means we will need to write a getGreeting() method in the class.

If this is the definition of your getGreeting() inside the class,

public String getGreeting()
{
    return 'Hi ' + contact.FirstName + ' how have you been?  Please update the following.';
}

you will then get an error message that looks like this:

System.SObjectException: SObject row was retrieved via SOQL without querying the requested field: Contact.FirstName

Most of us think that as soon as we call the getRecord() method, then the entire record is loaded into the contact variable.  However if you have an object with 5000 fields, and you only need to use 2 or 3 in your Visualforce pages, do you think it really makes sense to load all 5000 fields for that record into the contact variable?  Probably not.

Then what can you do?  The easiest way is to do add this into your Visualforce page:
<apex:outputText value="{!contact.FirstName}"/>

If you say, but I don't want to display the contact name twice!  Then you can simply change this to:
<apex:outputText value="{!contact.FirstName}" rendered="false" />

By adding rendered="false", that will hide this variable.  Or perhaps use <apex:variable> instead, such as:
<apex:variable value="{!contact.FirstName}" var="contactFName" />

When you load this Visualforce page, the standard controller will scan through your Visualforce page and knows that you need this field, and when making the getRecord() method call, it automatically and implicitly adds this FirstName field into the SOQL.

Remember, if you want to use any field in your extension via the getRecord() method, you need to let it be known by making sure that the field exists in your Visualforce page!

(Another alternative is to write your own SOQL in your Apex class, but why make an extra SOQL call when you can avoid it?)