Condition on child object from parent object model

That is very interesting. 

If checkinRecords is a parent to Checkin.  And you want to limit the attachments under checkin - I don’t believe you need any conditions on the child relationship.  We’re automatically going to associate the attachments with the appropriate Checkin record.  And it can only associated with one “chekinRecords” record. 

But I’m sure I’m missing somthing.