Combine TanStack Query Results In Angular: A How-To Guide

by Alex Johnson 58 views

In modern web development with Angular, managing data fetching and caching efficiently is crucial. TanStack Query (formerly React Query) has emerged as a popular library for handling these tasks with ease. This article will explore a common scenario: how to combine the results of two separate TanStack Query queries into a single object for use in your Angular application, all while maintaining the independence of the original queries. This approach allows you to fetch data from multiple sources and aggregate it into a unified structure, making it easier to manage and display in your UI. We will dive deep into various methods, providing clear examples and explanations to help you implement the best solution for your needs.

Understanding the Challenge

When working with TanStack Query in Angular, you often encounter situations where you need to fetch data from multiple endpoints or data sources. Each of these data-fetching operations is typically handled by a separate query. However, for UI presentation or other logic within your application, it's frequently necessary to combine the results of these queries into a single, cohesive object. This presents a challenge: how do you merge these results efficiently and reactively while ensuring that each query remains independent for caching and refetching purposes?

Consider a scenario where you have two queries:

  • One fetches user details.
  • Another fetches the user's roles or permissions.

You might want to display the user's information along with their roles in a single UI component. To achieve this, you need to combine the data from both queries. However, you also want to ensure that:

  • If the user data changes, only the user query is refetched.
  • If the roles data changes, only the roles query is refetched.

This requirement for independence is crucial for optimizing performance and ensuring a smooth user experience. Combining query results naively can lead to unnecessary refetching and performance bottlenecks. Therefore, it's essential to adopt a strategy that allows you to merge the data while preserving the individual query's lifecycle and caching behavior.

The Scenario: User Data and Roles

Let's delve into a practical example to illustrate the problem and potential solutions. Imagine you are building an application where you need to display user information along with their roles. You have two separate API endpoints:

  • /api/user: Returns user details (e.g., ID, name, email).
  • /api/roles: Returns the user's roles (e.g., admin, editor, viewer).

You've implemented two TanStack Query queries in your Angular component:

roles = injectQuery(() => ({
 enabled: this.user.data(),
 queryKey: ['roles'],
 queryFn: () =>
 lastValueFrom(getRoles({ roles: this.user.data().roles })),
}));

user = injectQuery(() => ({
 queryKey: ['user'],
 queryFn: () => lastValueFrom(getUser()),
}));

Here:

  • user query fetches user details.
  • roles query fetches the user's roles, and it's enabled only when the user data is available (to ensure you have the user ID to fetch roles).

Now, in your UI, you want to display something like:

<div *ngIf="user.data() as user">
 Roles: {{ user.roles?.length }}
</div>

However, the challenge is that user.roles doesn't exist directly. You need to combine the results of the user and roles queries. You've explored using injectQueries and computed signals, but these approaches have limitations:

  • injectQueries doesn't directly provide a way to merge the results with a custom function.
  • Computed signals return null until both queries load, which might not be ideal for your UI.

The Initial Attempts and Their Limitations

Let's examine the initial attempts to solve this problem and understand their drawbacks. One common approach is to use injectQueries as shown in the original example:

userActive = injectQueries(() => ({
 queries: [this.user, this.roles],
 select: (user, roles) => this.toUser(user, roles),
}));

toUser(user, roles) {
 return { ...user, roles: roles };
}

While injectQueries seems promising, it's not designed for this specific use case. The select function in injectQueries doesn't receive the results of the individual queries; instead, it receives the combined query state, which makes it difficult to merge the data effectively. This approach falls short of providing a clean and efficient way to combine the results.

Another attempt involves using computed signals:

activeUser = computed(() => {
 const user = this.user.data();
 const roles = this.roles.data();
 if (!user) return;
 if (!roles) return;

 return this.toUser(user, roles);
});

This approach has a significant drawback: the computed signal returns null until both user and roles queries have successfully loaded. This can lead to a poor user experience, as the UI might display nothing until all data is available. Additionally, it doesn't leverage TanStack Query's caching and invalidation mechanisms as effectively as possible.

These initial attempts highlight the need for a more robust and TanStack Query-native solution to combine query results while preserving the independence and benefits of individual queries.

The Solution: Leveraging computed and Query Selectors

The most effective solution involves combining Angular's computed signals with TanStack Query's query selectors. This approach allows you to reactively derive a new object containing the combined data while ensuring that each query remains independent.

Here's how you can implement this solution:

  1. Define your individual queries:

    You already have your user and roles queries defined using injectQuery. These queries will fetch data independently and manage their own caching and invalidation.

  2. Create a computed signal to combine the results:

    Use Angular's computed function to create a signal that depends on the data from both queries. Inside the computed signal, you can merge the data into a single object.

activeUser = computed(() => {
 const user = this.user.data();
 const roles = this.roles.data();

 // Use optional chaining and nullish coalescing to handle cases where data is not yet available
 return user && roles ? { ...user, roles: roles } : null;
});

In this code:

  • We use computed to create a reactive signal activeUser that automatically updates whenever user.data() or roles.data() changes.
  • We access the data from each query using .data(). This returns the query's data or undefined if the query is still loading or has encountered an error.
  • We use optional chaining (?.) and nullish coalescing (??) to handle cases where one or both queries haven't completed yet. This prevents errors and ensures that the computed signal only returns a combined object when both user and roles have data.
  • We use the spread syntax (...) to create a new object containing the user data and then add the roles to it. This ensures that we don't modify the original query data.

  1. Use the combined object in your template:

    In your Angular template, you can now use the activeUser signal to access the combined data.

<div *ngIf="activeUser() as user">
 User Name: {{ user.name }}
 Roles: {{ user.roles?.length }}
</div>
<div *ngIf="user.isFetching || roles.isFetching()">
 Loading...
</div>
<div *ngIf="user.isError() || roles.isError()">
 Error fetching data.
</div>

Here:

  • We use *ngIf to conditionally render the user information based on the availability of activeUser(). This ensures that the UI only displays data when it's ready.
  • We use the as keyword to assign the result of activeUser() to a local variable user for easier access in the template.
  • We can also access the individual query states (user.isFetching, roles.isError(), etc.) to display loading indicators or error messages.

Advantages of this Solution

This approach offers several advantages:

  • Reactivity: The computed signal automatically updates whenever the data from either query changes, ensuring that your UI stays in sync with the latest data.
  • Independence: The user and roles queries remain independent, allowing TanStack Query to manage their caching and invalidation separately. This optimizes performance and prevents unnecessary refetching.
  • Flexibility: You can easily customize the logic for combining the data within the computed function. This allows you to handle different data structures and merging requirements.
  • Type Safety: TypeScript's type system helps ensure that the combined object has the correct structure and that you're accessing the data safely.
  • Granular Loading and Error States: Accessing user.isFetching and roles.isError() gives you the ability to display loading and error states for each individual query, providing a more fine-grained user experience.

Handling Initial Data Availability

One potential issue with this approach is that the activeUser signal might initially be null if either user.data() or roles.data() is undefined. This can cause the UI to flicker or display an empty state briefly. To address this, you can use the nullish coalescing operator (??) or optional chaining (?.) in your template to provide a default value or handle the case where the data is not yet available.

For example, you can modify your template like this:

<div *ngIf="activeUser() as user">
 User Name: {{ user.name ?? 'Loading...' }}
 Roles: {{ user.roles?.length ?? 'Loading...' }}
</div>

Here, we use the nullish coalescing operator to display "Loading..." if user.name or user.roles?.length is null or undefined. This provides a smoother user experience by indicating that the data is being fetched.

Alternative: Query Selectors for Granular Data Transformation

Another powerful feature of TanStack Query is query selectors. Selectors allow you to derive specific pieces of data from a query's result without triggering unnecessary re-renders. While they don't directly combine multiple queries, they can be used in conjunction with computed signals to achieve a similar outcome with more fine-grained control.

Here's how you can use query selectors in this scenario:

  1. Define your individual queries:

    As before, you have your user and roles queries defined.

  2. Create selectors for the data you need:

    Use the select option in injectQuery to create selectors that extract specific properties from the query results.

const userQuery = injectQuery(() => ({
 queryKey: ['user'],
 queryFn: () => lastValueFrom(getUser()),
 select: (data) => ({
 id: data?.id,
 name: data?.name,
 email: data?.email,
 }),
}));

const rolesQuery = injectQuery(() => ({
 enabled: this.user.data(),
 queryKey: ['roles'],
 queryFn: () => lastValueFrom(getRoles({ roles: this.user.data().roles })),    
 select: (data) => data?.roles,
}));

In this code:

  • We define a select function for the user query that extracts the id, name, and email properties from the user data. This ensures that only these properties are used in the UI, preventing unnecessary re-renders if other properties change.
  • We define a select function for the roles query that extracts the roles array.

  1. Create a computed signal to combine the selected data:

    Use Angular's computed function to create a signal that combines the selected data from both queries.

activeUser = computed(() => {
 const user = this.userQuery.data();
 const roles = this.rolesQuery.data();

 return user && roles ? { ...user, roles: roles } : null;
});

This is similar to the previous solution, but now we're using the selected data from each query.

  1. Use the combined object in your template:

    Your template remains the same as in the previous solution.

Advantages of Query Selectors

  • Granular Control: Selectors give you fine-grained control over which data is used in your UI. This can significantly improve performance by preventing unnecessary re-renders.
  • Data Transformation: Selectors allow you to transform the data before it's used in your UI. This can be useful for formatting data, filtering arrays, or performing other data manipulations.
  • Composability: Selectors can be composed together to create complex data transformations.

When to Use Query Selectors

Query selectors are particularly useful when:

  • You only need a subset of the data from a query.
  • You need to transform the data before it's used in your UI.
  • You want to optimize performance by preventing unnecessary re-renders.

In the context of combining query results, selectors can be used to pre-process the data from each query before it's combined in a computed signal. This can lead to a more efficient and maintainable solution.

Conclusion

Combining the results of multiple TanStack Query queries in Angular requires a thoughtful approach to ensure reactivity, independence, and performance. By leveraging Angular's computed signals and TanStack Query's query selectors, you can effectively merge data from different sources into a single object for use in your UI. The combination of computed signals and query selectors provides a flexible and efficient way to manage data dependencies and transformations in your Angular applications using TanStack Query. This approach not only solves the problem of combining data but also promotes best practices for data fetching and management in modern web development.

Remember to choose the solution that best fits your specific needs and application requirements. Consider the complexity of your data transformations, the level of granularity you need, and the performance implications of each approach. By understanding the strengths and weaknesses of each technique, you can build robust and efficient Angular applications with TanStack Query.

For further exploration and best practices, refer to the official TanStack Query documentation available at https://tanstack.com/query/v5. This resource provides in-depth information on all aspects of TanStack Query and can help you master data fetching in your Angular applications.