error message show if cityname is already exist on vfpage

Question

want to show error message that if cityname is duplicate then show error message cityname already exists! whats i'm doing wrong anyone please tell

When saving record from cities tab and if dupicate city it showing error Kudos!!

when saving record from visualforce page it's not showing error message :(

class code:

 public pageReference save(){
    try{
        upsert cityNames;
    }catch(Exception e){
        String error = e.getMessage();
        ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR,error));
    }

    return new pageReference('/apex/CityControllerPage').setRedirect(true);

}        

---

Trigger code

set<String> cityName = new set<String>();

     for(City_Name__c  ct : Trigger.New){
           cityName.add(ct.name);    
     }

     List<City_Name__c> cList = [SELECT id, Name FROM City_Name__c where name IN : cityName];
 if(Trigger.isInsert){
        for(City_Name__c  ct : Trigger.New){
             if(ct.name!=null && cList.size()>0){
                 ct.name.addError('City Name already Exits');
             }
        }
    }    

---

Use page messages on vfpage

<apex:page controller="Ciler" tabStyle="Contact">

<apex:pagemessages ></apex:pagemessages>
<apex:form >
    <apex:pageBlock >
        <apex:pageBlockButtons location="top" >
            <apex:commandButton action="{!save}" value="Save"/>
        </apex:pageBlockButtons>

        <apex:pageBlockSection title="City Input">
            <apex:inputField value="{!cityNames.name}"/>
            <apex:inputField value="{!cityNames.Pin__c}"/>
        </apex:pageBlockSection>
    </apex:pageBlock>

Answer 1

Since you are creating city names upon looping through the Trigger.New, so you can execute the query and check the size of list.

 set<String> cityName = new set<String>();

 for(City_Name__c  ct : Trigger.New)
 {
       if(Trigger.isInsert)
       cityName.add(ct.name);    
 }
 List<AggregateResult> lst = 
                    [SELECT Name, COUNT(Id) 
                    FROM City_Name__c
                    WHERE name IN : cityName
                    GROUP By Name
                    HAVING COUNT(Id)>0];

   if(cList.size()>0){
       Trigger.new[0].addError('City Name already Exist');
    }

The above error is applicable for any matching found.

If you want to throw errors showing which specific city name matching with the result, then you need to loop through the list and show the error.

List<AggregateResult> lst = 
                    [SELECT Name, COUNT(Id) 
                    FROM City_Name__c
                    WHERE name IN : cityName
                    GROUP By Name
                    HAVING COUNT(Id)>0];

List<String> lstStr = new List<String>();                   
for (AggregateResult ar : lst)  {
    lstStr.add((String) ar.get('Name'));
}
Trigger.new[0].addError(String.join(lstStr,',') + ' already exist');

To return error message, use return null in catch block in controller method.