Firestore doesn't yet support collection group querying

[Atif-Mahmud]

So, I sent a message to the Firestore staff to fix this issue and they are working on it.

Here is the response:

Hey Atif,

Thank you for reaching out. This is definitely in our road map and is currently being worked on. However, I'm not able to provide any ETA for when it will be available. I will make sure to share your feedback with the team, the only suggestion for now is to keep an eye on our release notes an our blog to get more information about this.

To answer you question why this is not yet available, Firestore is a new product that will continue growing and improving, comments from the community like this one help us on this endeavor for which we are very grateful.

Cheers,
Gerardo

So, in the meanwhile, Deals will have to be a top-level collection, and each deal will have a document reference property which points to the Firestore doc that represents its respective business.