Anonymous code to run a bulk update

Question

I have three objects, namely Tests, Grant, and Grant Contracts.

Here in Tests object I have given a lookup to Grant and Grant Contract. I have also given a lookup in Grant Contract to the Grant object. In the lookup fields in all the test records, I have the grant record id.

My aim is to update all the grant contract fields in the test records, and these grant contracts should be of the parent grant object. So if I have a Tests record with Grant G and Grant Contract GC, I should get the tests record's grant contract field filled with the GC child record present in the Grant G.

I am planning to use anonymous code for updating as there is a bulk amount of records. Please provide an input on how to start. I have started this way.

       List<Test__c> TestList =[Select id,name,Grant__c from Test__c where Grant__c!= null];
    System.debug('Test==>'+TestList);
    System.debug('TestList==>'+TestList.size());
    Set<Id> grantId = new set<Id>();
    Set<Id> grantContractId = new set<Id>();
    
    For(Test__c t :TestList) {
    grantId.add(t.Grant__c);
    }
    System.debug('grantId==>'+grantId);
    System.debug('grantId Set==>'+grantId.size());




***Modified code and debug logs after the valuable answers from the comments.***




List<Test__c> TestList =[Select id,name,Grant__c,Grant_Contract__c from Test__c where Grant__c!= null];
System.debug('Test==>'+TestList);
System.debug('TestList==>'+TestList.size());

Map<Id, Grant_Contract__c> grantContractRecordsById = new Map<Id, Grant_Contract__c>();
System.debug('grantContractRecordsById');
    for(Test__c record: [
    SELECT Grant__c,Grant_Contract__c
    FROM Test__c
    WHERE Grant__c!=null AND Grant_Contract__r.Grant_Name__c=null
    LIMIT 10000])
    System.debug('record>=='+record);
        {
  grantContractRecordsById.put(
    record.Grant_Contract__c,
    new Grant_Contract__c(
      Id=record.Grant_Contract__c,
      Grant_Name__c=record.Grant__c
    )
  );
}
update grantContractRecordsById.values();  

Answer 1

That code would go something like this:

// Keep a collection of Grant_Contract__c records by Id, in case of duplicates
Map<Id, Grant_Contract__c> grantContractRecordsById = new Map<Id, Grant_Contract__c>();
// SOQL for loop 
for(Test__c record: [
    SELECT Grant__c,Grant_Contract__c 
    FROM Test__c 
    WHERE Grant__c != null AND 
          Grant_Contract__c != null AND 
          Grant_Contract__r.Grant__c = null
    LIMIT 10000]) {
  // Put a Grant_Contract__c record in the collection for updates.
  // Storing by Id ensures we don't get duplicates.
  grantContractRecordsById.put(
    record.Grant_Contract__c, 
    new Grant_Contract__c(
      Id=record.Grant_Contract, 
      Grant__c=record.Grant__c
    )
  );
}
// Save the changes to the database.
update grantContractRecordsById.values();

Be aware there's a 50k row limit for queries and 10k row limits for direct DML operations, which means you may need to run this script multiple times and/or adjust the limits.