How to Efficiently Query Firebase Chats Based on Participant Presence?

How to Query Chats Based on Participant's Presence

Problem Statement:

Given a Firebase database structure representing chat conversations with multiple participants, how can we efficiently query for chats that include a specific participant?

Current Query (Not Working):

return this.af.database.list('chats', {
    query: {
        orderByChild: 'participants',
        equalTo: username, // How to check if participants contain username
    }
});

Issues with the Current Query:

• The data structure uses an array to represent participants, which allows duplicate entries.
• Firebase only allows indexing on fixed paths within objects, making it challenging to filter on nested values like participants/username.

Recommended Solution: Invert the Index

To optimize the query, it's recommended to invert the index structure by flattening the data and pulling up the chatroom relationships to the user level. This involves:

• Storing a set of chat room IDs for each user under userChatrooms/{username}.
• Using a value of true in the set to represent that the user is a participant in the chat room.

Revised Data Structure:

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

Updated Query:

ref.child('userChatrooms').child(username)

Benefits of Inverted Index:

• Allows efficient retrieval of the list of chat rooms for a specific user using a single query.
• Supports querying for chat rooms based on participant presence without the need for dynamic indexing.
• Reflects the app's functionality more directly by modeling the user's perspective.

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