How to set a Lookup Field via a Salesforce Trigger - triggers

Question

I want to set a Lookup Field using a salesforce apex trigger, but I keep getting an error:
System.StringException: Invalid id:
I have a custom object called Job__c. It has a custom pick list for accounts: Acct__c.
A user will populate Acct__c as John Deer, the trigger should add John Deer to the Account__c lookup field.
Here is the trigger:
trigger UpdateAccounts on Job__c (before insert) {
for (Job__c obj: trigger.new){
obj.Account__c = obj.Acct__c; //Exception is thrown here
}
Exception thrown:
System.StringException: Invalid id:
I tried something different:
List <Job__c> opListInsert = new List<Job__c>();
List <Job__c> opListUpdate = new List<Job__c>();
if(trigger.isInsert){
for(Job__c op:trigger.New){
if(op.Acct__c != Null){
op.Account__c = op.Acct__c;
opListInsert.add(op);
}
}
}
else if(trigger.isUpdate){
for(Job__c op:trigger.New){
if(op.Acct__c != Null && op.Acct__c !=trigger.oldMap.get(op.id).Acct__c){
op.Account__c = op.Acct__c;
opListUpdate.add(op);
}
}
}
That code throws:
Error:Apex trigger UpdateAccounts caused an unexpected exception, contact your
administrator: UpdateAccounts: execution of BeforeUpdate caused by:
System.StringException: Invalid id:
What am I doing wrong that it tells me it's an invalid ID?

Answer 1

Account__c is a look-up field, you cannot assign a string (the picklist field) to it. Instead you have to assign it an account id.
I am not sure why you're using the account names in the picklist field, but if you want to continue doing that then there's a simple solution that the account names are unique.
Get the id for the account selected in the query for the id, something like this:
list<account> acclist = [select id from account where name In yourNameList];
And then in the trigger reference the id
obj.account__c = accList[0].id;
Make use of maps , do not add the soql's inside for loop

Answer 2

You have to set your field with the ID of the object you want it to link to.
The salesforce browser page has a handy dandy tool that lets you put in incomplete information into the lookup field, and it fills it in for you automatically. This does not happen when values are assigned manually with an apex statement. You can only set the value to a valid and existing ID of the object the lookup was set to link to.
This code will grab the ID given name and do what you wanted:
trigger UpdateAcct on Job__c (before insert, before update) {
List<String> Accounts = new List<String>();
for (Job__c obj: trigger.new){
Accounts.add(obj.Acct__c);
}
list<Account> acctlist = [select Name from account where Name in :Accounts];
if (acctlist.size() > 0 ){
for (Integer i = 0; i < Trigger.new.size(); i++)
{
if (Trigger.new[i].Acct__c != null)
{
Trigger.new[i].Account__c = acctlist[i].ID;
}
else
{
Trigger.new[i].Account__c = null;
}
}
}
}