Resolving undefined Issues in Angular Material MatTable with Chained Firebase Queries

Discover how to fix the `undefined` error in Angular Material MatTable when using chained AngularFire queries by creating a composite index in Firestore.
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Fixing the undefined Error in Angular Material MatTable with Chained Firebase Queries

If you're developing an Angular application and utilizing Angular Material's MatTable alongside Firebase's Firestore, you may encounter an annoying issue when performing chained queries. Specifically, you might see the error: ***ERROR TypeError: Cannot read properties of undefined (reading 'length')***. This often suggests that the filteredData array is not being set correctly when combining multiple queries. Let’s take a closer look at this issue and explore how to resolve it.

The Problem

You are likely trying to execute a query that combines multiple Firestore filters using .where() and .orderBy(), as shown below:

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

When you utilize this setup, the filteredData variable is returning as undefined, causing issues down the line when your Angular Material table attempts to rely on this data. The underlying issue is related to how Firestore manages composite queries and indexing for performance.

Understanding the Issue

While the error message seems straightforward, the solution might not be immediately apparent. Here is what happens when you perform multiple queries with Firestore:

Firestore requires composite indexing to combine multiple fields in a single query. When this is not set up correctly, the query can return unexpected results, which in this case leads to filteredData being undefined.

In your particular case, when you use a single query without chaining (.where() or .orderBy() individually), it works as expected because those cases do not require composite indexing.

The Solution: Create a Composite Index

To resolve the error you're facing, you need to create a composite index in your Firestore database. Follow these steps:

Go to Firestore Database: In the Firebase console, navigate to your Firestore database.

Select the Indexes Tab: Find the "Indexes" option in the left sidebar.

Create a Composite Index: Click on the "Create Index" button. You'll need to add fields that correspond to your query:

For this case, add the year field and the lastName field.

Set Index Options: Define the index configurations as needed (e.g., ascending/descending).

Build and Deploy: Save the index and wait for Firestore to build it, which can take a few moments.

After creating the necessary composite index, your chained query should work without returning any undefined errors.

Conclusion

In summary, when you encounter an issue with filteredData returning as undefined in your Angular Material MatTable while trying to perform chained Firestore queries, the problem is likely rooted in the absence of a required composite index. By following the steps outlined above, you can create this index and resolve the issue effectively.

For any developers working with Angular and Firebase, understanding how indexing works in Firestore is crucial for optimizing queries and ensuring your application's data handling remains smooth and error-free.

Now you're ready to tackle queries in your application with confidence! Happy coding!