Add Custom Error Message to VF Page

Question

I am trying to add a custom error message to a VF page. My controller saves, but when I save my VF page without the checkbox field being checked, it saves anyway and bypasses the error condition in my controller. What am I doing wrong?

VF Page:

<apex:page standardController="Quote__c" extensions="QuoteExtController">
<apex:messages/>
<apex:form >
    <apex:commandButton value="Save" action="{!save}"/>
    <apex:pageblock>
        Quote Name: <apex:InputField value="{!Quote__c.Name}"/>
        Quote Check: <apex:InputField value="{!Quote__c.Must_Be_Checked__c}"/>
    </apex:pageblock>
</apex:form>
</apex:page>

Controller Extension:

public with sharing class QuoteExtController {
    private quote__c quote;

    public QuoteExtController(ApexPages.StandardController stdController) {
        this.quote = (Quote__c)stdController.getRecord();
    }

    public PageReference onSave() {
        if(quote.Must_Be_Checked__c==TRUE) {
            ApexPages.Message myMsg = new  ApexPages.Message(ApexPages.Severity.ERROR,'Check field');
            ApexPages.addMessage(myMsg); 
            return null;
        } else if (quote.Id != null) {
            insert quote;
        }

        return null;
    }
}

Answer 1

Your command button is calling the standard controller save. You need to use your custom action in the save button, shown below:

<apex:commandButton value="Save" action="{!onSave}"/>

Answer 2

It's because you're using Quote__c in your page, and not your quote page property.

Change your VF markup to use this:

<apex:pageblock >

  Quote Name: <apex:InputField value="{!quote.Name}"/>
  Quote Check: <apex:InputField value="{!quote.Must_Be_Checked__c}"/>
</apex:pageblock>

And finally, make sure you're calling your custom onSave method and not the standard Save method.