How to get List of All Child Records for Given Parent Records in .csv File using APEX

script_to_find_All_child_records

string parentObjectName ='Account';//Specify Object API Name
string parentRecordId = '0019xxxxxxx7MqU'; //Specify record Id
//to store childRelationShipName
Map<integer,List<string>> childObjectmap = new Map<integer,List<string>>(); 
//to store inner query string for child objects
Map<integer,List<string>> childObjectQStringmap = new Map<integer,List<string>>(); 
Map<String, Schema.SObjectType> schemaMap = Schema.getGlobalDescribe();
for(String ss1: schemaMap.keyset()){
    Schema.SObjectType objToken=schemaMap.get(ss1);
    if(ss1.equalsignorecase(parentObjectName)){
        //find details about sobject
        Schema.DescribeSObjectResult objDescribe=objToken.getdescribe();
        List<Schema.ChildRelationship> childRelationshipList = objDescribe.getChildRelationships();
        system.debug('*********childRelationshipList size:'+childRelationshipList.size()); 
        for(Schema.ChildRelationship ss:childRelationshipList){
            if(ss.getRelationshipName()!=null){
                string RelationshipName=ss.getRelationshipName();
				string childObjName = string.valueof(ss.getChildSObject());
                //Check if child object is isQueryable() or not
                Schema.DescribeSObjectResult childObjDescribe = schemaMap.get(childObjName).getdescribe();
                system.debug('*******childObjName:'+childObjName);
                //system.debug('*********isRestrictedDelete():'+ss.isRestrictedDelete());
                //system.debug('*********RelationshipName:'+RelationshipName);
                if(childObjDescribe.isQueryable()){
                    if(childObjectmap.size()==0){
                        childObjectmap.put(1,new List<string>());
                        childObjectmap.get(childObjectmap.size()).add(RelationshipName);
                        childObjectQStringmap.put(1,new List<string>());
                        childObjectQStringmap.get(childObjectQStringmap.size()).add('(select id from '+ RelationshipName + ' )');
                    }else{
                        if(childObjectmap.get(childObjectmap.size()).size()<20){
                            childObjectmap.get(childObjectmap.size()).add(RelationshipName);
                            childObjectQStringmap.get(childObjectQStringmap.size()).add('(select id from '+ RelationshipName + ' )');
                        }else{
                            childObjectmap.put(childObjectmap.size()+1,new List<string>());
                            childObjectmap.get(childObjectmap.size()).add(RelationshipName);
                            childObjectQStringmap.put(childObjectQStringmap.size()+1,new List<string>());
                            childObjectQStringmap.get(childObjectQStringmap.size()).add('(select id from '+ RelationshipName + ' )');
                        }
                    }    
                }else{
                    system.debug('******'+childObjName+' object is not querable.');
                }
            }
        }
    }
}
system.debug('***childObjectmap.size():'+childObjectmap.size());
system.debug('***childObjectQStringmap.size():'+childObjectQStringmap.size());
string header = 'Related Object name , Records deatils \n';
string finalstr = header ;
try{
    for(integer i:childObjectmap.keyset()){
        string qString = 'Select id,'+ string.join(childObjectQStringmap.get(i),',') +' from ' +parentObjectName+ ' where Id=:parentRecordId LIMIT 1';
        system.debug('***qString:'+qString);
        sObject records = database.query(qString);
        for(string childObjectRec:childObjectmap.get(i)){
            string childRecordId ='';
            for(sobject ch:records.getSObjects(childObjectRec)){
                childRecordId = childRecordId +'  '+ ch.get('Id');
            }
            //string recordString= childObjectRec + ',' + records.getSObjects(childObjectRec) + '\n';
            string recordString= childObjectRec + ',' + childRecordId + '\n';
            finalstr = finalstr + recordString;
        }
    }
}catch(exception ex){
    system.debug('******exception:'+ex.getmessage());
}
string OrgURL = URL.getSalesforceBaseURL().toExternalForm();
Messaging.EmailFileAttachment csvAttc = new Messaging.EmailFileAttachment();
blob csvBlob = Blob.valueOf(finalstr);
string csvname= parentObjectName + '(' + parentRecordId + ') Related child records.csv';
csvAttc.setFileName(csvname);
csvAttc.setBody(csvBlob);
Messaging.SingleEmailMessage email =new Messaging.SingleEmailMessage();
String[] toAddresses = new list<string> {UserInfo.getUserEmail()};
String subject =parentObjectName + '(' + parentRecordId + ') Related child records  Report - Org URL:'+OrgURL;
email.setSubject(subject);
email.setToAddresses( toAddresses );
email.setPlainTextBody(subject);
email.setFileAttachments(new Messaging.EmailFileAttachment[]{csvAttc});
Messaging.SendEmailResult [] r = Messaging.sendEmail(new Messaging.SingleEmailMessage[] {email});