Apex Class + Trigger to populate Lookup field

Question

I have an Apex Class:

    public with sharing class Lookup
    {
    public static void findOpportunities(List<Custom_Object__c> records)
    {
        Set<Id> OppsIds = new Set<Id>();
        for (Custom_Object__c record : records) OppsIds.add(record.custom_field__c);

        Map<String, Opportunity> Opps = new Map<String, Opportunity>();
        for (Opportunity Opp : [
            SELECT Id, custom_field_2__c FROM Opportunity
            WHERE custom_field_2__c IN : OppsIds
        ]) Opps.put(Opp.Name, Opp);

        for (Custom_Object__c record : records)
            if (Opps.containsKey(record.custom_field__c))
                record.Opportunity__c = Opps.get(record.custom_field__c).Id;
    }
}

And a trigger:

trigger UpdateOpportunityLookup on Custom_Object__c (after insert, after update) 
{

    if (trigger.isAfter)
    {
        if (trigger.isInsert)
        {
            Lookup.findOpportunities(trigger.new);
        }
    }
}

The OLIs are being sent from another system on this custom object, as related records. There is lookup field from the related records to the Opportunity object that I am trying to populate.

The Class is meant to query if the custom_field__c on the custom_object__c has the same value as custom_field_2__c and populate the lookup with the correct Opportunity.

I have managed to save it successfully, but I am getting an error message:

Apex script unhandled trigger exception by user/organization / caused by: System.StringException: Invalid id: 895

External entry point Trigger.UpdateOpportunityLookup: line 8, column 1

and it stops the OLIs from being sent to SF.

Both fields are Text Data Type and contain numeric numbers.

The custom_field_2__c on the Opportunity I am querying to have the same value as custom_field__c is External ID = ticked.

Answer 1

This variable cannot be a Set<Id>:

    Set<Id> OppsIds = new Set<Id>();

because the values you're receiving are not Salesforce Ids; they're arbitrary strings. Change the collection type to Set<String>.

The exception you're seeing is thrown when you attempt to create an Id value from something that is not an Id.

Additionally, this logic is incorrect:

    for (Opportunity Opp : [
        SELECT Id, custom_field_2__c FROM Opportunity
        WHERE custom_field_2__c IN : OppsIds
    ]) Opps.put(Opp.Name, Opp);

Your Map should be keyed on custom_field_2__c, not Name.