How to Efficiently Query Firebase or Firestore for Chats Containing a Specific Participant?

Firebase Query to Find Chats Containing a Participant

Background:

The data structure under consideration represents a chat system where chats have participants and chat items. The goal is to query the "chats" table to retrieve all chats that have a specific participant based on a given username string.

Initial Attempt:

The provided code attempts to use the orderByChild and equalTo methods to query the "chats" table by the "participants" child, specifying the username to find matches. However, this approach is limited because:

• Participants are stored as an array: This data structure does not accurately reflect the fact that participants in a chat should be unique.
• Indexing limitations: Firebase requires pre-defined indices for querying nested paths, which can become cumbersome when dealing with dynamic data.

Inverted Index Approach:

To address these limitations, it is recommended to invert the data structure by creating a "userChatrooms" node that maps users to the chat rooms they participate in. This allows for efficient filtering by user:

userChatrooms: {
  john: {
    chatRoom1: true,
    chatRoom2: true
  },
  puf: {
    chatRoom1: true,
    chatRoom3: true
  }
}

Query using Inverted Index:

With the inverted index structure, finding all chats for a user becomes straightforward:

ref.child("userChatrooms").child("john")

Cloud Firestore Alternative:

Cloud Firestore provides better support for this type of query with its array-contains operator. This allows direct filtering of documents that contain a specific value in an array:

Query query = firestore.collection("chats")
                    .whereArrayContains("participants", username);

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