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Discover how to maintain array type information when using Hasura with Apollo GraphQL and PostgreSQL by utilizing custom scalar handling techniques.
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How to Properly Convert Hasura/PostgreSQL Scalar Arrays in Apollo GraphQL Using Codegen

GraphQL has revolutionized how we interact with APIs, and with tools like Apollo and Hasura, the process has become even more efficient. However, when integrating PostgreSQL with Hasura and leveraging Apollo's code generation capabilities, developers might encounter issues, particularly regarding scalar array fields. This post aims to address the problem of converting PostgreSQL scalar arrays into GraphQL types accurately, allowing you to keep your APIs both robust and intuitive.

The Problem: Scalar Array Representation in GraphQL

When using Hasura to generate a GraphQL interface for PostgreSQL tables, you may have noticed a discrepancy in how scalar arrays are represented. One common scenario revolves around having a column defined as integer[] in your PostgreSQL table. When executing the command apollo schema:download, the following output is generated for that column:

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

Unfortunately, as shown in the response, this output fails to convey critical information: the fact that this field is an array. This becomes problematic when you run apollo codegen:generate, as the scalar type may be incorrectly transformed into a simple String or simply _int4, especially if the --passthroughCustomScalars flag is applied.

The Solution: Leveraging the Underscore Prefix

It turns out that the underscore (_) prefix in the type name can be an indicator of an array type. While there may be other situations leading to such a type name, you can safely assume the following for your use case:

int4 corresponds to Integer.

_int4 indicates Integer[] (an array of integers).

Thus, by recognizing this pattern, you can effectively configure your Apollo SDK to manage this scalar array properly without losing type information.

Step-by-Step Approach

Here’s how you can implement a solution:

Use --passthroughCustomScalars Flag:

When executing apollo codegen:generate, include the --passthroughCustomScalars flag. This ensures that the custom scalar types are preserved during code generation.

Define Type Aliases:

Create type aliases for your Apollo SDK configuration that associates _int4 with Integer[]. This tells your application how to interpret the GraphQL response correctly.

Implement JSON Encode/Decode Methods:

Ensure that you define appropriate methods for encoding and decoding JSON for your newly aliased scalar type. This enhances compatibility and avoids runtime issues.

Example Code Snippet

Here’s a brief example of how you might define your custom scalar type:

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

Conclusion

Encountering issues with GraphQL schemas can be frustrating, especially when integrating complex data types like PostgreSQL scalar arrays. By understanding how the underscore prefix can signal arrays, you can effectively configure Apollo code generation to properly manage these types.

This workaround may seem somewhat rough around the edges, but it allows you to maintain type safety and clarity within your GraphQL APIs. With this approach, you can continue to leverage the power and efficiency of Hasura and Apollo, transforming your database interactions into effective API calls.

Now, you are equipped to handle these scalar array types with confidence!