A way to combine data from two collections when you can't use a collectionGroup because you're not supposed to be able to access all the data from one of the lists!

GoalListRepository.kt

package com.aniketkadam.dogether.goals

import androidx.lifecycle.LiveData
import androidx.lifecycle.MediatorLiveData
import com.aniketkadam.dogether.auth.FIREBASE_USER_ID
import com.aniketkadam.dogether.di.data.PRIVATE_GOALS_LOCATION_NAME
import com.aniketkadam.dogether.di.data.PUBLIC_GOALS_LOCATION_NAME
import com.aniketkadam.dogether.goals.data.GoalsListLiveData
import com.google.firebase.firestore.CollectionReference
import com.google.firebase.firestore.QueryDocumentSnapshot
import javax.inject.Inject
import javax.inject.Named

class GoalListRepository @Inject constructor(
    @Named(PUBLIC_GOALS_LOCATION_NAME) val publicGoalsLocation: CollectionReference,
    @Named(PRIVATE_GOALS_LOCATION_NAME) val privateGoalsLocation: CollectionReference,
    @Named(FIREBASE_USER_ID) private val userId: String?
) {

    private val queryConversion: (QueryDocumentSnapshot) -> GoalWithMetadata =
        { snapshot -> GoalWithMetadata(snapshot.id, snapshot.toObject(Goal::class.java)) }

    fun getOwnGoals(): LiveData<List<GoalWithMetadata>> = CombinedLocations(
        GoalsListLiveData(privateGoalsLocation, queryConversion),
        GoalsListLiveData(publicGoalsLocation, queryConversion) {it.whereEqualTo(UID_PARAM, userId)}
    )

    fun getAllGoals(): LiveData<List<GoalWithMetadata>> = CombinedLocations(
        GoalsListLiveData(privateGoalsLocation, queryConversion),
        GoalsListLiveData(publicGoalsLocation, queryConversion)
    )

}

/**
 * Combines several sources of goals list live data into a single one.
 * However, it can't deal with infinite lists and paging will need to be handled separately
 */
private class CombinedLocations(vararg goalsListLiveData: GoalsListLiveData<GoalWithMetadata>) : MediatorLiveData<List<GoalWithMetadata>>() {

    private var privateSource : MutableMap<Int, List<GoalWithMetadata>> = mutableMapOf()
    private fun refresh() {
        postValue(privateSource.values.reduce { acc, list -> acc.plus(list) })
    }

    init {
        goalsListLiveData.forEachIndexed { index, currentData ->
            addSource(currentData) {
                privateSource.put(index, it)
                refresh()
            }
        }
    }
}