Скачать или смотреть Sorting Firestore Documents: GeoHash Limitations and Workarounds

Discover effective strategies for sorting documents in Firestore with GeoHash and additional fields. Learn how to retrieve top-rated restaurants nearby.
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Sorting Firestore Documents: GeoHash Limitations and Workarounds

In the world of database management, organizing and retrieving data efficiently is crucial. If you've ever tried to filter and sort data from Firestore based on geographic information along with additional fields, you might have faced some challenges. This post will address the common issue of sorting documents by GeoHash and another field — specifically, getting the top-rated restaurants based on a rating count.

The Problem: Sorting Challenge in Firestore

Imagine you have a dataset of 10,000 restaurants, each identified by a GeoHash which represents their geographic location. You want to sort these restaurants based not only on their location but also on their rating count in descending order—the higher the rating, the better!

Here's a sample scenario:

You have a Firestore query that looks like this:

[[See Video to Reveal this Text or Code Snippet]]

You expect to receive the top-rated restaurants between given geographic bounds represented by hash values tsp0 and tsp3 with a limit of 2 documents returned.

Expected vs. Actual Results

Expected Results: [item3, item2] (assuming item3 has the highest count within the geographic bounds)

Actual Results: [item2, item1]

This discrepancy raises an important question: Is this a limitation in Firestore, or is there a workaround? Let’s dive deeper into the mechanics behind this issue.

Understanding the Limitation

The reason for the unexpected sorting behavior in the results lies in how Firestore processes the query. Firestore orders the documents first by GeoHash and only afterward by count. Hence, the count field influences the ordering only among documents that share the same GeoHash value.

Breakdown of Why the Query Didn't Work as Expected:

Order of Sorting: Firestore respects the order in which fields are specified in the orderBy clause. Since the first sort is by geoHash, it dictates the primary sorting mechanism.

Field Similarity: In your sample documents, item1 and item2 share the same geoHash, which means their order is determined solely by the count. In essence, because item3 has a different geoHash, it is excluded from the initial sorting selection, leading to unexpected results in a multi-field sort.

Possible Solutions and Workarounds

While this limitation exists due to the nature of how Firestore indexes data, there are a couple of strategies you can implement to achieve the desired results:

1. Filter by GeoHash and Sort in Application Code

One effective workaround is to conduct your query focused solely on geoHash first, then filter and sort the remaining results by count in your application. This involves:

Running the original query:

[[See Video to Reveal this Text or Code Snippet]]

Storing the results in an appropriate data structure (like a list) within your app.

Sorting that data structure based on the count value using your application logic.

2. Merging Count into GeoHash

While more complex, another workaround might be to adjust the dataset itself. By merging the count into the geoHash to create a unique composite field that prioritizes both criteria, you could potentially maintain sorted order. However, this method could complicate your data structure significantly and may not be advisable unless necessary.

Conclusion

Sorting documents in Firestore based on both GeoHash and an additional field like count poses challenges due to the ordering limitations inherent in Firestore's query system. While the ideal scenario you envisioned might not be feasible directly within the database, leveraging application-level sorting after performing the query is a practical workaround. This appr