How to use Schema class in Trigger to update a related child object feild?

Question

Requirement : create a field called 'ExternalRef' on all child objects of Opportunity Object write a trigger on Opportunity that updates the new field on all child objects of opportunity to 'Rajsheakr'+System.now();

Trigger :

Trigger RelatedObjectsFeildsUpdate on Opportunity (after update) {

Map < Id,  Opportunity > mapoppty = new Map < Id, Opportunity >();
for(Opportunity oopty : trigger.new)
    mapoppty.put(oopty.Id, oopty);


List<quote> listquote = new List<quote>();
listquote = [ select id,ExternalRef__c,Opportunity.name,Opportunity.id from quote  where Opportunity.id =: mapoppty.keySet() ];
if ( listquote.size() > 0 ) {
    for ( quote q : listquote ) {
        q.ExternalRef__c = 'raj shekar'+system.now() ;

    }
    update listquote;
}


list<payment__c> lstpymnt = new list<payment__c>();
lstpymnt = [select id,ExternalRef__c,payment__r.name from payment__c where payment__c=: mapoppty.keySet()];
system.debug('lstpymnt'+lstpymnt);
if ( lstpymnt.size() > 0 ) {
    for ( payment__C p : lstpymnt ) {
        p.ExternalRef__c = 'raj shekar'+ system.today();

    }
    update lstpymnt;
}

}

I acheived like this but i have to do it using schema and update the ExternalRef on child objects with 'raj shekar'+system.now();

is it possible to do that generically.

schema class what i think is useful : for getting related child object

 Schema.DescribeSObjectResult R = Opportunity.SObjectType.getDescribe();
 for (Schema.ChildRelationship cr: R.getChildRelationships()) 
 {
  system.debug('====child object==='+cr.getChildSObject());
 }

for getting feilds (manually giving object):

 Map<String, Schema.SObjectField> fieldsMap = 
 Schema.SObjectType.Quote.fields.getMap();

  for (Schema.SObjectField field : fieldsMap.values())
   {
  System.Debug(field.getDescribe().getName());
  }  

Answer 1

The ChildRelationship itself has these methods:

getChildSObject()
Returns the token of the child sObject on which there is a foreign key back to the parent sObject.

getField()
Returns the token of the field that has a foreign key back to the parent sObject.

getRelationshipName()
Returns the name of the relationship.

that provide enough information for you to construct the dynamic code you need:

Set<Id> oppIds = mapoppty.keySet();
DescribeSObjectResult dsr = Opportunity.SObjectType.getDescribe();
for (ChildRelationship cr : dsr.getChildRelationships()) {
    SObject[] sobs = Database.query(''
        + ' select Id, ExternalRef__c'
        + ' from ' + cr.getChildSObject()
        + ' where ' + cr.getField() + ' in :oppIds'
    );
    for (SObject sob : sobs) {
        sob.put('ExternalRef__c', 'raj shekar'+ system.today());
    }
    update sobs;
}