How to Define a Custom Merge Function for InMemoryCache in GraphQL

Learn how to resolve cache warnings in GraphQL using Apollo Client by defining a custom merge function for your InMemoryCache.
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How to Define a Custom Merge Function for InMemoryCache in GraphQL

When working with GraphQL and Apollo Client, it's common to encounter issues related to cache management. One such problem is the warning: "Cache data may be lost when replacing the parts field of a Query object." This warning often triggers when there's an attempt to modify cached data without properly merging existing and incoming objects in the cache. Let’s dive deep into solving this issue with a custom merge function.

Understanding the Problem

In this case, you might have a GraphQL query that retrieves parts of some entities. When you use a mutation to delete a specific part, Apollo Client tries to update the cache with the new data. If the cache isn't configured properly for merging, you will receive warnings about potential data loss.

Here’s a simplified example of what happens in your cache when you try to delete a part using a mutation:

Existing cached parts: [{ "__ref": "Part:53" }, { "__ref": "Part:55" }, { "__ref": "Part:56" }, { "__ref": "Part:57" }, { "__ref": "Part:58" }]

Incoming parts after deletion: [{ "__ref": "Part:53" }, { "__ref": "Part:55" }, { "__ref": "Part:56" }, { "__ref": "Part:57" }]

Because of this mismatch, Apollo Client raises a warning indicating that data may be lost.

The Solution: Custom Merge Function

To address this warning, you need to define a custom merge function within the Apollo Client setup. This function dictates how to merge the existing cache with incoming data. Here’s a step-by-step breakdown of how to implement this.

Step 1: Configuring Apollo Client

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

Step 2: Understanding the Code

typePolicies: This property allows you to define how specific types or fields should be handled in the cache.

merge function: The merge function takes two parameters, existing (cached data) and incoming (newly fetched data). In our case, we return a combined array that includes parts from both arrays.

Step 3: Adjust Your Mutation

In relation to your mutation, if you’re using the update option to manipulate cache, consider simplifying it or even removing the evict and modify operations, unless absolutely necessary.

Your mutation should ideally refresh the relevant query, allowing the new merge function to operate efficiently:

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

Benefits of the Custom Merge Function

Implementing this custom merge function provides several advantages:

Prevent Data Loss: Ensures no existing parts in the cache are lost when new parts are added or removed.

Improved Performance: By intelligently merging cache data, you improve the efficiency of your application.

Reduced Warnings: Eliminates cache-related warnings during runtime, leading to a cleaner development experience.

Conclusion

Managing cache in GraphQL, particularly using Apollo Client, can sometimes be tricky. However, by defining a custom merge function in your InMemoryCache configuration, you can effectively resolve warnings related to data loss and enhance the overall functionality of your application. If you find yourself facing similar issues, remember to revisit your cache policies and tailor them to your data-specific needs.

With these steps, you should have a solid understanding of how to effectively manage cache data within your GraphQL Apollo Client applications.