How to exclude an element from a Firestore query?

问题:

I have a collection of users and I want to query all users from the database and display them in a RecyclerView except one, mine. This is my db schema:

users [collection]
  - uid [document]
     - uid: "fR5bih7SysccRu2Gu9990TeSSyg2"
     - username: "John"
     - age: 22
  - //other users

How to query the database like so:

String uid = FirebaseAuth.getInstance().getCurrentUser().getUid();
Query q = db.collection("users").whereNotEqualTo("uid", uid);

So I need this query object to be passed to a FirestoreRecyclerOptions object in order to display all the other users in RecyclerView.

Is this even possible? If not, how can I solve this? Thanks!

Edit:

options = new FirestoreRecyclerOptions.Builder<UserModel>()
        .setQuery(query, new SnapshotParser<UserModel>() {
            @NonNull
            @Override
            public UserModel parseSnapshot(@NonNull DocumentSnapshot snapshot) {
                UserModel userModel = documentSnapshot.toObject(UserModel.class);
                if (!userModel.getUid().equals(uid)) {
                    return userModel;
                } else {
                    return new UserModel();
                }
            }
        }).build();

回答1:

After days and days of struggling with this issue, I finally found the answer. I could not solve this without the help of @Raj. Thank you so much @Raj for the patience and guidance.

First off all, according to the answer provided by @Frank van Puffelen in his answer from this post, I stopped searching for a solution that can help me pass two queries to a single adapter.

In this question, all that I wanted to achieve was to query the database to get all the users except one, me. So because we cannot combine two queries into a single instance, I found that we can combine the result of both queries. So I have created two queries:

FirebaseFirestore db = FirebaseFirestore.getInstance();
Query firstQuery = db.collection("users").whereLessThan("uid", uid);
Query secondQuery = db.collection("users").whereGreaterThan("uid", uid);

I'm having a UserModel (POJO) class for my user object. I found not one, but two ways to solve the problem. The first one would be to query the database to get all user objects that correspond to the first criteria and add them to a list. After that, query the database again and get the other user objects that correspond to the second criteria and add them to the same list. Now I have a list that contains all the users that I need but one, the one with that particular id from the queries. This is the code for future visitors:

firstQuery.get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
    @Override
    public void onComplete(@NonNull Task<QuerySnapshot> task) {
        List<UserModel> list = new ArrayList<>();
        if (task.isSuccessful()) {
            for (DocumentSnapshot document : task.getResult()) {
                UserModel userModel = document.toObject(UserModel.class);
                list.add(userModel);
            }

            secondQuery.get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                @Override
                public void onComplete(@NonNull Task<QuerySnapshot> task) {
                    if (task.isSuccessful()) {
                        for (DocumentSnapshot document : task.getResult()) {
                            UserModel userModel = document.toObject(UserModel.class);
                            list.add(userModel);
                        }

                        //Use the list of users
                    }
                }
            });
        }
    }
});

The second approach would be much shorter because I use Tasks.whenAllSuccess() like this:

Task firstTask = firstQuery.get();
Task secondTask = secondQuery.get();

Task combinedTask = Tasks.whenAllSuccess(firstTask, secondTask).addOnSuccessListener(new OnSuccessListener<List<Object>>() {
        @Override
        public void onSuccess(List<Object> list) {
            //This is the list that I wanted
        }
});

回答2:

Firestore doesn't support not equal to operation. So you need to filter the data at the client side. Since in you case you only have one extra item you can filter it out.

For that you may need to build your own recycler implementation where when adding data to recycler adapter data layer, you restrict the data when ever it matches your != condition.

I haven't explored recycler implementation firebase provided so I cannot say it supports data manipulation to adapter data or not.

Here is a good resource to start implementing recycler view : https://www.androidhive.info/2016/01/android-working-with-recycler-view/

回答3:

According to the official firestore documentation:-

Cloud Firestore does not support the following type of query:

Queries with a != clause. In this case, you should split the query into a greater-than query and a less-than query. For example, although the query clause where("age", "!=", "30") is not supported, you can get the same result set by combining two queries, one with the clause where("age", "<", "30") and one with the clause where("age", ">", 30).

If you are using FirestoreRecyclerAdapter then FirestoreRecyclerOptions will directly accepts the query using setQuery() method and hence not allows you to perform client side filtering.

If you try to apply filters in onBindViewHolder() while setting the data that might results in empty items in the recycler view. In order to resolve that refer Method 2.

So, the possible solution to your problem would be to create an integer field in your users collection under every document. Eg:-

users [collection]
  - uid [document]
     - uid: "fR5bih7SysccRu2Gu9990TeSSyg2"
     - username: "John"
     - age: 22
     - check: 100

In this I have created a 'check' variable whose value is 100. So, put value of 'check' in all other documents as less than 100. Now, you can easily make a query that finds documents with check<100 as:-

Query q = db.collection("users").whereLessThan("check", 100);

This will retrieve all your documents except the one you don't want. And while setting the data you can set other parameters skipping the check variable.

Method 2 (Client Side Filtering)

We can apply a check in onBindViewHolder() method that if the retrieved uid matches with current user uid then set the height of Recycler view as 0dp. As:-

ViewUserAdapter.java

public class ViewUserAdapter extends FirestoreRecyclerAdapter<User, ViewUserAdapter.ViewUserHolder>
{
    String uid;
    FirebaseAuth auth;

    public ViewUserAdapter(@NonNull FirestoreRecyclerOptions<User> options)
    {
        super(options);
        auth = FirebaseAuth.getInstance();
        uid = auth.getCurrentUser().getUid();
    }

    @Override
    protected void onBindViewHolder(@NonNull ViewUserHolder holder, int position, @NonNull User model)
    {
        DocumentSnapshot snapshot =  getSnapshots().getSnapshot(position);
        String id = snapshot.getId();

        if(uid.equals(id))
        {
            RecyclerView.LayoutParams param = (RecyclerView.LayoutParams)holder.itemView.getLayoutParams();
            param.height = 0;
            param.width = LinearLayout.LayoutParams.MATCH_PARENT;
            holder.itemView.setVisibility(View.VISIBLE);

        }
        else
        {
            holder.tvName.setText(model.name);
            holder.tvEmail.setText(model.email);
            holder.tvAge.setText(String.valueOf(model.age));
        }
    }
}

回答4:

The simplest solution would be to use a PagedListAdapter and create a custom DataSource for the Firestore queries. In the DataSource the Query can be transformed into an Array or ArrayList in which you can easily remove your item before adding the data to the method callback.onResult(...).

I used a similar solution to process data after a Firestore query in order to filter and sort by a time attribute, and then re-sort by a quality score attribute in the client before passing the data back in to callback.onResult(...).

Documentation

• Google: Build your own data sources
• Codepath: Paging Library Guide

Data Source Sample

class ContentFeedDataSource() : ItemKeyedDataSource<Date, Content>() {

override fun loadBefore(params: LoadParams<Date>, callback: LoadCallback<Content>) {}

override fun getKey(item: Content) = item.timestamp

override fun loadInitial(params: LoadInitialParams<Date>, callback: LoadInitialCallback<Content>) {
    FirestoreCollections.contentCollection
            .collection(FirestoreCollections.ALL_COLLECTION)
            .orderBy(Constants.TIMESTAMP, Query.Direction.DESCENDING)
            .whereGreaterThanOrEqualTo(Constants.TIMESTAMP, DateAndTime.getTimeframe(WEEK))
            .limit(params.requestedLoadSize.toLong())
            .get().addOnCompleteListener {
                val items = arrayListOf<Content?>()
                for (document in it.result.documents) {
                    val content = document.toObject(Content::class.java)
                    items.add(content)
                }
                callback.onResult(items.sortedByDescending { it?.qualityScore })
            }
}

override fun loadAfter(params: LoadParams<Date>, callback: LoadCallback<Content>) {
    FirestoreCollections.contentCollection
            .collection(FirestoreCollections.ALL_COLLECTION)
            .orderBy(Constants.TIMESTAMP, Query.Direction.DESCENDING)
            .startAt(params.key)
            .whereGreaterThanOrEqualTo(Constants.TIMESTAMP, DateAndTime.getTimeframe(WEEK))
            .limit(params.requestedLoadSize.toLong())
            .get().addOnCompleteListener {
                val items = arrayListOf<Content?>()
                for (document in it.result.documents) {
                    val content = document.toObject(Content::class.java)
                    items.add(content)
                }
                val sortedByQualityScore = ArrayList(items.sortedByDescending { it?.qualityScore })
                callback.onResult(sortedByQualityScore)
                sortedByQualityScore.clear()
            }
}
}

回答5:

Simpler and earlier client-side filtering (when you add items to your list):

1. Get the current user's ID by using Firestore's standard method.
2. Get the name of the doc for all the users in your user collection.
3. Before adding the user to your RecyclerView list, check that the user it is about to add to your list is not the current user.

When done is this way, you can use the "not equals" method on the client side and not get into any Firestore issues. Another benefit is that you don't have to mess with your adapter or hide the view from a list-item you didn't want in the recycler.

public void getUsers(final ArrayList<Users> usersArrayList, final Adapter adapter) {

    CollectionReference usersCollectionRef = db.collection("users");

    Query query = usersCollectionRef
            .whereEqualTo("is_onboarded", true);

    query.get()
            .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                @Override
                public void onComplete(@NonNull Task<QuerySnapshot> task) {
                    if (task.isSuccessful()) {
                        for (QueryDocumentSnapshot document : task.getResult()) {

                            final String otherUserID = document.getId();

                             FirebaseUser user = mAuth.getCurrentUser();
                             String currentUserID = user.getUid();

                            if (!otherUserID.equals(currentUserId)) {

                              usersArrayList.add(new User(otherUserID));
                              adapter.notifyDataSetChanged(); //Ensures users are visible immediately
                                            }
                                        } else {
                                            Log.d(TAG, "get failed with ", task.getException());
                                        }
                                    }
                                });
                            }

                        }
                    } else {
                        Log.d(TAG, "Error getting documents: ", task.getException());
                    }
                }
            });
}