Unable to Order Data Chronologically Using Cloud Firestore

Question

I understand that Cloud Firestore does not automatically order data sequentially. I have created a value, which is essentially the epoch converted to hours. I want to order data by this value for my Query:

    Query queryStore = FirebaseFirestore.getInstance()
            .collection(POLLS_LABEL)
            .orderBy("epoch", Query.Direction.DESCENDING);

I am finding that my data is not being ordered as I am hoping. Unsure if I need to add a special annotation to my code, but I would imagine it is as simple as the above. Below is my model:

@com.google.firebase.firestore.IgnoreExtraProperties
public class Poll {

private String question;
private String image_URL;
private String user_ID;
private String display_name;
private int vote_count;
private Long epoch;
private long trend_score;
@ServerTimestamp
private Date date;

private ArrayList<String> answers;

public Poll() {
}

public Poll(String Question, String Image_URL, ArrayList<String> answers, int vote_count, String UserID, String DisplayName, Long epoch, long trend_score, Date date
            ) {
    this.question = Question;
    this.image_URL = Image_URL;
    this.answers = answers;
    this.vote_count = vote_count;
    this.user_ID = UserID;
    this.display_name = DisplayName;
    this.epoch = epoch;
    this.trend_score = trend_score;
    this.date = date;
}

public String getUser_id() {
    return user_ID;
}

public String getDisplay_name() {
    return display_name;
}

public String getQuestion() {
    return question;
}

public void setQuestion(String question) {
    this.question = question;
}

public String getImage_URL() {
    return image_URL;
}

public Long getEpoch() {
    return epoch;
}

public void setEpoch(Long epoch) {
    this.epoch = epoch;
}

public void setUser_id(String user_id) {
    this.user_ID = user_id;
}

public void setDisplay_name(String display_name) {
    this.display_name = display_name;
}

public void setImage_URL(String image_URL) {
    this.image_URL = image_URL;
}

public Integer getVote_count() {
    return vote_count;
}

public void setVote_count(Integer vote_count) {
    this.vote_count = vote_count;
}

public long getTrend_score() {
    return trend_score;
}

public void setTrend_score(long trend_score) {
    this.trend_score = trend_score;
}

@Exclude
public Map<String, Object> toMap() {
    HashMap<String, Object> result = new HashMap<>();
    result.put("question", question);
    result.put("image_URL", image_URL);
    result.put("vote_count", 0);
    result.put("user_ID", user_ID);
    result.put("display_name", display_name);
    result.put("epoch", epoch);
    result.put("trend_score", trend_score);
    return result;
}

Creation:

final Poll poll = new Poll(mCreatePollQuestion.getText().toString(), resultImageURL, mPollAnswers, 0, ID, displayName, currentEpoch, intialQS, FieldValue.serverTimestamp());

Adding to Firestore:

      mStoreBaseRef.collection(POLLS).add(poll).addOnCompleteListener(new OnCompleteListener<DocumentReference>() {
                @Override
                public void onComplete(@NonNull Task<DocumentReference> task) {
                    if (task.isSuccessful()) {
                        DocumentReference docRef = task.getResult();
                        String key = docRef.getId();
                        Log.v("KEY", key);
                        Toast.makeText(getApplicationContext(), key, Toast.LENGTH_LONG).show();
                        CollectionReference pollAnswerRef = mStoreBaseRef.collection("Polls").document(key).collection("answers");
                        //TODO: need to add answers
                        for (int i = 0; i < mPollAnswers.size(); i++){
                            pollAnswerRef.document(String.valueOf(i + 1)).set((poll.answerConvert(mPollAnswers, i)), SetOptions.merge());
                        }

                    }
                }

Answer 1

You are guessing right, you should use an annotation but your epoch object should not be of type Long but Date. So in your Poll class, please change the following line of code:

private Long epoch;

to

@ServerTimestamp
private Date date;

I named that date object as date because the type of object is Date and is not and epoch anymore. When you create an object of Pool class, there is no need to set the date. Firebase servers will read your date field as it is a ServerTimestamp (see the annotation) and it will populate that filed with the server timestamp accordingly.

Another solution to add the date would be to use a Map and a FieldValue.serverTimestamp() like this:

Map<String, Object> map = new HashMap<>();
map.put("date", FieldValue.serverTimestamp());
docRef.update(map).addOnCompleteListener(new OnCompleteListener<Void>() {/* ... */}