Using a SUM aggregate from a CI to the related Expenses

[JohnnieGross]

Hello, everyone.

I have set up a custom CI Class called Master Service Agreement (MSA) under the Document class.

The cmdb_ci is already related to the Expense object, where the Related List for expenses can be added.  I have added the related Expense to my Master Service Agreement.

I have added expenses to a MSA with the appropriate amounts and would like for those amounts to "roll up" via a SUM aggregate to that particular MSA.  

Ideally, the aggregate would populate the Cost field of the MSA (via the column coming from cmdb_ci) or a custom field I have that filters out expenses by fiscal year.  I was able to set up that custom relationship and it is working just fine.

However, I cannot seem to find any mechanism or function where I can do an aggregate from the child (Expenses) ot the Parent (Master Service Agreement CI).  This seems like an everyday-garden-variety thing that one should be able to do.  I thought I'd use the Glide Aggregate function, but cannot find a use-case to match a roll-up from a child object to it's parent.

I am attaching some screenshots to match my description here.

Thanks for any help!

John