angular - Firebase query if child of child contains a value -

the structure of table is:

• chats
• --> randomid
• -->--> participants
• -->-->--> 0: 'name1'
• -->-->--> 1: 'name2'
• -->--> chatitems

etc

what trying query chats table find chats hold participant passed in username string.

here have far:

 subscribechats(username: string) {     return this.af.database.list('chats', {         query: {             orderbychild: 'participants',             equalto: username, // how check if participants contain username         }     });  } 

a few problems here:

• you're storing set array
• you can index on fixed paths

set vs array

a chat can have multiple participants, modelled array. not ideal data structure. each participant can in chat once. using array, have:

participants: ["puf", "puf"] 

that not have in mind, data structure allows it. can try secure in code , security rules, easier if start data structure implicitly matches model better.

my rule of thumb: if find writing array.contains(), should using set.

a set structure each child can present @ once, naturally protects against duplicates. in firebase you'd model set as:

participants: {   "puf": true } 

the true here dummy value: important thing we've moved name key. if i'd try join chat again, noop:

participants: {   "puf": true } 

and when you'd join:

participants: {   "john": true,   "puf": true } 

this direct representation of requirement: collection can contain each participant once.

you can index fixed paths

with above structure, could query chats in with:

ref.child("chats").orderbychild("participants/john").equalto(true) 

the problem require define index on `participants/john":

{   "rules": {     "chats": {       "$chatid": {         "participants": {           ".indexon": ["john", "puf"]         }       }     }   } } 

this work , perform great. each time new joins chat app, you'll need add index. that's not scaleable model. we'll need change our data structure allow query want.

invert index - pull categories up, flattening tree

second rule of thumb: model data reflect show in app.

since looking show list of chat rooms user, store chat rooms each user:

userchatrooms: {   john: {     chatroom1: true,     chatroom2: true   },   puf: {     chatroom1: true,     chatroom3: true   } } 

now can determine list of chat rooms with:

ref.child("userchatrooms").child("john") 

and loop on keys each room.

you'll have 2 relevant lists in app:

• the list of chat rooms specific user
• the list of participants in specific chat room

in case you'll have both lists in database.

chatroomusers   chatroom1     user1: true     user2: true   chatroom2     user1: true     user3: true userchatrooms   user1:     chatroom1: true     chatroom2: true   user2:     chatroom1: true   user2:     chatroom2: true 

i've pulled both lists top-level of tree, since firebase recommends against nesting data.

having both lists normal in nosql solutions. in example above we'd refer userchatrooms inverted index of chatroomsusers.