Trigger to calculate number of times a pick list field has been used

Question

On field lead source of opportunity object, if currently there are 4 records for the value 'Web', and the user tries to create or update an existing record the trigger should throw an error stating that there can not be more than 4 records for the value web. Till now i have got this

set<ID> userIdset = new set<ID>();
set<string> stageName = new set <string> ();
List<Opportunity> stageList = new List <Opportunity> ();
if (Trigger.isInsert) 
{
    Schema.DescribeFieldResult fieldResult = Opportunity.stageName.getdescribe();
    List<schema.PicklistEntry> schemaPickList = fieldResult.getPickListvalues();
    System.debug('Schema picklist'+schemaPicklist);
    System.debug('stageList'+'\t'+stageList);

    for (Opportunity o1 : Trigger.new) 
    {

    }
}

if (Trigger.isUpdate) 
{
    for (Opportunity o2:trigger.new)
    {
        if (o2.stageName != Trigger.oldMap.get(o2.Id).stageName) 
        {
            System.debug('Update trigger fired');
            system.debug('The old status was'+Trigger.oldMap.get(o2.Id).stageName);
            userIdset.add(o2.Id);
        }
    }
}

Answer 1

My best guess is that you want to make sure that no account has more than 4 opportunities that were created via leads that came from the web.

Disclaimer: the following is intentionally not copy/paste code and meant as a guide to get you started. Use it as a guide and if you run into any specific issues you can ask another question for more assistance.

trigger oppTriggerTest on Opportunity (before update, before insert) {
    // FIrst step is to get account ids
    Set<id> accountIds = new Set<Id>();

    for (Opportunity opp : trigger.new) {
        if (opp.leadSource = 'Web') { // Make sure we only look at opps that we care about
            accountIds.add(opp.Id);
        }
    }

    // Let's make the database do as much of the work as possible
    List<AggregateResult> ars = [
        SELECT Count(Id) numOfWeb, AccountId
        FROM Opportunity
        WHERE AccountId IN :accountIds
            And LeadSource = 'Web'
        GROUP BY AccountId
        HAVING Count(Id) >= 4
    ];

    // Lets make a map that's easier to work with
    Map<Id, Integer> accountToCount = new Map<id, Integer>();

    for (AggregateResult ar : ars) {
        accountToCount.put(ar.get('AccountId'), ar.get('numOfWeb'));
    }

    // Now, lets add an error to opps that violate the rule
    for (Opportunity opp : trigger.new) {
        if (accountToCount.containsKey(opp.accountId)) {
            opp.addError('Too many web opps for the given account');
        }
    }
}