public static string FormatFieldsForQuery(string objectName,string prefix){
    if(string.isNotBlank(prefix)){
        if(prefix.endsWith('__c'))
            prefix=prefix.replace('__c','__r.');
        if(!prefix.endsWith('.'))
            prefix+='.';
    }
    string fields = '';
    sObjectType objectType=Schema.getGlobalDescribe().get(objectName);
    if(objectType==null)
        return fields;
    for(string f :objectType.getDescribe().fields.getMap().keySet())
        fields+=prefix+f+',';
    return fields.removeEnd(',');
}