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Learn how to avoid duplicate keys in JSON while generating Terraform resources with the `for_each` loop. This guide walks you through effective strategies to handle redundancy in your JSON data.
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How to Skip Duplicate Keys from JSON When Using for_each in Terraform

Handling JSON files with duplicate keys can be a significant challenge, especially when you're trying to generate Terraform resources dynamically. If you’ve ever encountered a scenario where your JSON data contains repeated keys, you know how frustrating it can be. In this post, we’ll explore a straightforward solution to this common problem, ensuring you can process your JSON data in a clean and efficient manner.

Understanding the Problem

Let’s start by looking at a brief example to clarify the situation. Consider the following JSON snippet:

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As you can see, the key "21-10" appears twice within the "my-ptr-zone" object. This presents a problem when you try to generate Terraform resources using a loop with for_each, as a typical JSON decoder would typically discard all but the last occurrence of the duplicate keys.

The Solution: Using Terraform's jsondecode Function

When you use the jsondecode function in Terraform, it inherently manages duplicate keys in a specific way. Here’s what you need to know:

Behavior of jsondecode: Terraform's jsondecode will discard all but the last definition of a duplicate key found in a JSON object. Thus, if you have repeated keys, the final entity in the list will prevail.

Example of Using jsondecode

Here’s how your example JSON is processed by Terraform:

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In this output, only the last occurrence of the key "21-10" is preserved after decoding, which effectively means the duplicate issue is resolved—at least in the context of the produced Terraform value.

Implementation in Resource Definition

You can use this decoded JSON directly in your Terraform resource definitions. Here’s how you might write a resource for "aws_route53_zone" using the for_each loop:

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Important Notes

Source of JSON Files: While handling duplicates in Terraform can be straightforward with jsondecode, it's still advisable to address duplicates at the source of your JSON data whenever possible. This helps prevent potential confusion and keeps your data clean.

Testing Your Solution: Always validate your Terraform configuration after implementing these adjustments to ensure everything is working as expected. You can use the terraform plan command to preview changes before applying them.

Conclusion

Dealing with duplicate keys in JSON can seem daunting, but with Terraform’s jsondecode function, you have a reliable way to ensure that only the last defined value is considered. By following the steps outlined in this guide, you can efficiently manage your JSON data and avoid redundant resource definitions in your Terraform configurations.

If you face any challenges with duplicate keys or have questions about Terraform, feel free to leave a comment or reach out for additional support!