What's the best way I can query channels by a sub field?

[jaejoong-y]

Our channel has custom fields and a field has sub fields.
ex)
adInfo: {adId: xxx, adTitle: xxx}

I tried to query channels by a sub field with a dot notation as it seems that's the way MongoDB query does. But it doesn't seem to be working.
Filters.eq("adInfo.adId", "xxx")

I have no idea what field name I have to put.
Would you be able to let me know how I can achieve this?