Trigger : Throw an error if we try to update an Account with contacts

Recently I got a requirement on Triggers.Trigger should trow an error if we try to update an Account which contains child contacts.
Update should be successful if Account does not contains any child Contacts.
I felt some how difficulty in writing this trigger by considering governor limits and optimizing the use of collections but I managed to write the trigger successfully.. After writing this trigger I wanted to share it with others and here is the trigger.

trigger AccountsWithContacts on Account (before update) {
    
    Set<Id> accWithContacts = new Set<Id>();
    for(Contact c: [Select Id,Name,AccountId from Contact Where AccountId In: Trigger.NewMap.KeySet()])
    {
        accWithContacts.add(c.AccountId);
    }
    
    for(Account acc:Trigger.New)
    {
        if(accWithContacts.Contains(acc.Id))
            acc.addError('could not update Account with contacts');
    }

}

Difference between apex:message,apex:messages,apex:pageMessage and apex:pageMessages in salesforce

We have different components that supports displaying messages in a visual force page like <apex:message>,<apex:messages>,<apex:pageMessage> and <apex:pageMessages>. Let us see how each component behaves.

Lets take some sample code to understand each component. We have not included any message components in the VF.

<apex:page controller="MyRegisterCon2" sidebar="false" docType="HTML-5.0"   >

<apex:form >
  <apex:pageBlock >
  <apex:pageblockSection columns="1" >
  <apex:inputtext value="{!FirstName}" label="First Name"  />
  <apex:inputtext value="{!LastName}" label="Last Name" />
  <apex:input value="{!Jdate}" label="DOB" type="date" />
  <apex:inputtext value="{!Phone}" label="Phone"  id="phone"/>
  </apex:pageblockSection>
  <apex:pageBlockButtons >
  <apex:commandButton value="submit" action="{!save}" />
  </apex:pageBlockButtons>
  </apex:pageBlock>
  </apex:form>
</apex:page>

public with sharing class MyRegisterCon2 {

    public Date Jdate { get; set; }
   
    public Integer Phone { get; set; }
    
    public String LastName { get; set; }

    public String FirstName { get; set; }
    
    public PageReference save() {
    
        return null;
    }
       

}

Though phone and DOB are not strings it is not showing any errors when you click on submit.

<apex:message>:

This  component is used to display warning or error message for a specific component.

Change visualforce to following code.

<apex:page controller="MyRegisterCon2" sidebar="false" docType="HTML-5.0"   >

<apex:form >
  <apex:pageBlock >
  <apex:pageblockSection columns="1" >
  <apex:message for="phone" />
  <apex:inputtext value="{!FirstName}" label="First Name"  />
  <apex:inputtext value="{!LastName}" label="Last Name" />
  <apex:input value="{!Jdate}" label="DOB" type="date" id="dob" />
  <apex:inputtext value="{!Phone}" label="Phone"  id="phone"  />

  </apex:pageblockSection>
  <apex:pageBlockButtons >
  <apex:commandButton value="submit" action="{!save}" />
  </apex:pageBlockButtons>
  </apex:pageBlock>
  </apex:form>

</apex:page>

In above code I have included  <apex:message for="phone" /> . Though we have given text for DOB and Phone fields the message is shown for only specific component.
 i.e. Phone.

<apex:messages>

Include above code in VF page it will display all the messages . If we observe styling was not applied to the messages.

<apex:page controller="MyRegisterCon2" sidebar="false" docType="HTML-5.0"   >

<apex:form >
  <apex:pageBlock >
  <apex:pageblockSection columns="1" >
  <apex:messages />
  <apex:inputtext value="{!FirstName}" label="First Name"  />
  <apex:inputtext value="{!LastName}" label="Last Name" />
  <apex:input value="{!Jdate}" label="DOB" type="date" id="dob" />
  <apex:inputtext value="{!Phone}" label="Phone"  id="phone"  />
  </apex:pageblockSection>
  <apex:pageBlockButtons >
  <apex:commandButton value="submit" action="{!save}" />
  </apex:pageBlockButtons>
  </apex:pageBlock>
  </apex:form>

</apex:page>

<apex:pageMessages>

The  above component provides the SF styling for the messages. 

<apex:page controller="MyRegisterCon2" sidebar="false" docType="HTML-5.0"   >

<apex:form >
  <apex:pageBlock >
  <apex:pageblockSection columns="1" >
  <apex:pageMessages />
  <apex:inputtext value="{!FirstName}" label="First Name"  />
  <apex:inputtext value="{!LastName}" label="Last Name" />
  <apex:input value="{!Jdate}" label="DOB" type="date" id="dob" />
  <apex:inputtext value="{!Phone}" label="Phone"  id="phone"  />
  </apex:pageblockSection>
  <apex:pageBlockButtons >
  <apex:commandButton value="submit" action="{!save}" />
  </apex:pageBlockButtons>
  </apex:pageBlock>
  </apex:form>

</apex:page>

<apex:pageMessage>

This component is used to dispaly custom messages with severity error,warning etc.  in the VF page.

<apex:page controller="MyRegisterCon2" sidebar="false" docType="HTML-5.0"   >

<apex:form >
  <apex:pageBlock >
  <apex:pageblockSection columns="1" >
  <apex:pageMessage summary="This is page message"  severity="warning" strength="3" />
  <apex:inputtext value="{!FirstName}" label="First Name"  />
  <apex:inputtext value="{!LastName}" label="Last Name" />
  <apex:input value="{!Jdate}" label="DOB" type="date" id="dob" />
  <apex:inputtext value="{!Phone}" label="Phone"  id="phone"  />
  </apex:pageblockSection>
  <apex:pageBlockButtons >
  <apex:commandButton value="submit" action="{!save}" />
  </apex:pageBlockButtons>
  </apex:pageBlock>
  </apex:form>

</apex:page>

Hope this will be helpful to understand differrent messages in VF.

Thanks,
Naveen.