How to insert Lookup field value in a Trigger?

Question

I am trying to build a after insert trigger on Contract object. Whenever user will create a Contract record trigger will generate/insert record of other object called Dim2. Dim2 have a lookup with Contract.

Here is my Trigger

trigger Contract2DIM2 on Contract (after insert) {
        if(Trigger.isInsert){
          for(Contract c : Trigger.New) {
            C2D2.insertDim2(c.ContractNumber);  
        }
}

Here is my class

public class C2D2 {
    public static void insertDim2(String Cnumber){
        try{
                c2g__codaDimension2__c a=new c2g__codaDimension2__c();
                a.Name=Cnumber;
                a.c2g__ReportingCode__c=Cnumber;
                a.Contract_Reference__c=Cnumber;
                insert a;             
        }catch(dmlexception e){
            system.debug('galti'+e.getMessage());
        }

    } 
}

It gives the following Error

Answer 1

There are couple of things which I think are not perfect when it comes to writing on Salesforce Platform. I will point out the usual.

1. Contact_Reference__c field on c2g__codaDimension2__c object is a lookup field . WHich means you can only put ID(15 or 18Digit valid SF record Id into it) As I can see you are inputting a number, which will give you exception

2. Secondly Your Trigger code is not bulkified. If someone tried to use Bulk load using data loader your trigger will just get into 150DML limits. You have to bulkify your code.

   trigger Contract2DIM2 on Contract (after insert) {
       if(Trigger.isInsert){
   
           C2D2.insertDim2(Trigger.New);   //Bulkifying trigger by passing List to the Service method
       }
   }
   

   Service class code:

   public class C2D2 {
   public static void insertDim2(List<Contract> newList){
   
       List<c2g__codaDimension2__c> tobeInsertedDim2 = new List<c2g__codaDimension2__c>();
       for(Contract c : List<Contract> newList) {
           tobeInsertedDim2.add(new new c2g__codaDimension2__c(Name=c.ContractNumber ,c2g__ReportingCode__c =c.ContractNumber,
               Contract_Reference__c =c.Id )); //Notice Contact Reference I am setting as contractId
       }
       try{
           insert tobeInsertedDim2;         
       }catch(dmlexception e){
           system.debug('galti'+e.getMessage());
       }
   
   } 
   }
   

Answer 2

Instead of passing a string into the class you should pass the contract object and then assign the id of the contract to the lookup field on the c2g__codaDimension2__c i.e. Contract_Reference__c

I have corrected your code here it should be

trigger Contract2DIM2 on Contract (after insert) {
        if(Trigger.isInsert){
          for(Contract c : Trigger.New) {
            C2D2.insertDim2(c);  
        }
}

public class C2D2 {
    public static void insertDim2(Contract ctrct){
        try{
                c2g__codaDimension2__c a=new c2g__codaDimension2__c();
                a.Name=ctrct.ContractNumber;
                a.c2g__ReportingCode__c=ctrct.ContractNumber;
                a.Contract_Reference__c=ctrct.ID;
                insert a;             
        }catch(dmlexception e){
            system.debug('galti'+e.getMessage());
        }
    } 
}

The problem is that the lookup field c2g__codaDimension2__c is expecting a "string" or "ID" but you are assigning it a number so you get a dml exception.

Please mark it as best answer if it helps you.