Sum field based upon the unique combination of 2 others

I'm searching for a way to accomplish the following:
I've got a date histogram interval that is by "day" and i'd like to add the parent_size into a sum only once even if the related associations.c occurs many times within that date histogram interval. Ideally I'd to be able to perform this query with any date histogram interval using the same "sum parent_size once for each unique associations.c per day" logic.

Below is an example document in the index I'm querying against:

    "associations": {
        "a": [
        "b": [
        "c": [
    "parent_size": 110,
    "id": "d5fe6216-7eb7-4d81-b3b2-eef28850b80d",
    "created_at": "2016-05-23T23:51:17.661Z"