Realtime Queries: Mastering Foreign Tables In Supabase

Hey everyone! 👋 Today, we're diving deep into a super cool topic: realtime queries with foreign tables in Supabase. I know, I know, it sounds a bit complex, but trust me, it's actually pretty manageable, and once you get the hang of it, you'll be querying data like a pro. This guide is tailored to help you understand how to fetch those related objects in real-time. Let's get started, shall we?

The Challenge: Realtime Data and Foreign Keys 🔑

So, the question at hand: How do we handle foreign key relationships in Supabase's realtime feature? As you guys know, when you have foreign keys set up in your database, you can link different tables together. For example, you might have a countries table and a cities table, where each city is related to a specific country. Now, the cool part is that you can query these related tables together! But, the original poster pointed out a crucial detail: how does this work in real-time? They correctly observed that the standard selectAsFlow function often only returns the ID of the related object, not the entire object itself. This is where we need to get a bit creative and find the perfect solution to query foreign tables in realtime.

Let's go back and explore the original problem. The issue is that the user wants to get the entire related object in a real-time query, and not just the ID. For example, let's say we have the following code:

val flow: Flow<List<Country>> = supabase.from(‘countries’).selectAsFlow(Country::id)
flow.collect {
    for (country in it) {
        println(country.name)
    }
}

This code is great for getting the Country objects in real time, but if you want to include data from related tables, like cities, you'll only receive the foreign key which isn't what the user expects. So, how can we solve this? Well, the solution involves understanding a couple of Supabase's features and a bit of Kotlin magic.

The Old Way:

If you've been working with Supabase for a while, you might be familiar with the following way to query related tables:

val columns = Columns.raw("""
    id,
    name,
    cities (
      id,
      name
    )
""".trimIndent())
val country = supabase.from("countries")
  .select(
    columns = columns
  )
  .decodeSingle<Country>()

This is a solid way to query all the data at once, but it doesn't do anything for realtime. The issue is that this is a single query and won't be updated. So, if your data gets updated, the application won't know. Now that we know that this cannot be used, we will have to use another approach to solve the main problem.

Realtime Queries: The Solution 💡

Here’s how we can tackle this. Since selectAsFlow might not directly support nested selections like the regular select with raw columns, we'll need a way to combine the realtime aspect with the ability to fetch related data. The recommended solution would be to make two different calls. One call to get the countries in real-time and one call for the cities which contains a country_id column.

// 1. Get countries in real-time
val countryFlow: Flow<List<Country>> = supabase.from("countries").selectAsFlow()

// 2. In each country, fetch related cities
countryFlow.collect {
    for (country in it) {
        // Fetch cities related to this country
        val cities = supabase.from("cities")
            .select("id, name") // Select the specific columns you need
            .filter("country_id", "eq", country.id.toString())
            .decodeList<City>()
    }
}

In the code above, the first part uses selectAsFlow to get the countries in real-time. In the second part of the code, when we receive the countries, we execute a new query with filter to get the cities related to the countries we are watching in real time.

Advantages of this approach

• Realtime Updates: Any changes in the countries table will trigger updates, and any changes in the cities table will trigger updates on the second query. This ensures that your application always has the most recent data.
• Clear Separation: The code is easy to read, and it separates concerns. Each step is very clear.
• Flexibility: You can decide exactly which columns of the related data you want to fetch.

Considerations and Optimizations 🚀

Now, let's talk about some things to keep in mind and how we can make things even better. I mean, we're already writing awesome code, but let's go the extra mile!

• Performance: Executing queries inside a collect block can be a bit slow. Instead, you can optimize this by using a flatMapConcat or a similar operator to merge the data streams. This way, the cities will be fetched in parallel. This will provide a significant performance improvement.
• Error Handling: It's super important to include error handling. You should use try-catch blocks and handle potential errors when fetching related data. Also, consider the supabase.from(...).execute() function with a try-catch.
• Data Transformation: Sometimes, you might need to transform the data before displaying it in your UI. Use Kotlin's map to transform your data. For example, if you want a Country with a list of City objects, create a data class to store the desired result.

Real-World Use Cases 🌍

Alright, let's look at some cool examples of how you could use this in your projects. We're going to dive into the practical side of this, so you can see how it applies to real-world scenarios. We'll be using the techniques we've discussed to pull live data from related tables.

• E-commerce: Imagine you have a product and its related reviews. With this technique, you can display products with their latest reviews in real-time. Any new review will automatically appear on your app.
• Social Media: Display a user's posts along with their comments in real-time. When a user creates a new comment, the changes will be automatically reflected.
• Chat Applications: In chat applications, it's essential to display user messages and their details in real-time. With the approach mentioned above, you can fetch all the messages with the user information in an easy and reliable way.

Conclusion: Realtime Foreign Table Queries 🎉

So, there you have it! We've covered the ins and outs of querying foreign tables in real-time using Supabase. We talked about why it's important, how to do it, and the things to keep in mind. I hope this helps you guys out! I hope this helps you build some truly amazing apps.

Remember to optimize your queries, handle errors, and make use of Kotlin's powerful features. If you have questions or want to share your own experiences, drop a comment below. Happy coding, and have fun building awesome apps with Supabase! 🎉