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Dive into the world of Terraform with this guide on using `for_each` effectively for nested structures. Learn how to manage resources dynamically and efficiently!
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This video is based on the question https://stackoverflow.com/q/67490398/ asked by the user 'Harry Leboeuf' ( https://stackoverflow.com/u/7568316/ ) and on the answer https://stackoverflow.com/a/67491038/ provided by the user 'Martin Atkins' ( https://stackoverflow.com/u/281848/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Mastering Terraform Nested for_each: A Comprehensive Guide

When embarking on your journey with Terraform, one of the most daunting challenges can be understanding how to manipulate complex data structures, particularly when using the for_each feature. If you're just starting out, you might find yourself grappling with how to set up nested for_each statements. This post will walk you through this process using a real-world example, enabling you to harness the full power of Terraform effectively.

The Challenge: Creating a Nested for_each

Suppose you have a scenario where you're working with Azure, and you're trying to set up multiple storage resources while assigning roles to Azure Active Directory (AAD) groups. You may have a data structure defined in your .tfvars file, like this:

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

You already implemented a basic module to create storage resources using a for_each, but now you want to go a step further – you need to create roles and assign AAD groups dynamically for each storage account. The question you find yourself asking is: How do I structure this to achieve my goal?

The Solution: Setting up Nested for_each

Understanding for_each Requirements

Before diving into code, it’s essential to understand the requirements for using for_each:

Collection: You need a collection that represents each resource instance you want to declare.

Unique Keys: You must have unique string keys for each element in the collection that are known at planning time.

Given our example, you already have your role definitions outlined in locals.st_blob_contributor_role_aadgroups. However, it requires an additional transformation to convert it into a suitable map format for for_each.

Transforming the Data Structure

To modify your local definition, you’ll implement the following code snippet, which creates a unique key for each entry by combining the storage key (st_key) and the role base key (rbac_key):

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

Implementing Nested for_each in Your Module

With your transformed data structure, you can now use this in your module definition. Below is an example module where for_each is employed successfully:

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

Accessing Data Inside the Module

Inside your module, you can reference the current iteration’s attributes easily using each.value. For example, if you need to get the display name or role, you can use:

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

Ensuring Uniqueness

It's crucial that the combination of st_key and rbac_key yields a unique identifier. Since they were derived from different sources, this approach minimizes the risk of conflicts.

Conclusion: Empowering Your Terraform Infrastructure

By following this structured approach, not only can you set up complex Terraform configurations, but you can also gain confidence in how to manipulate data dynamically. As you venture further into Terraform, mastering such constructs will significantly enhance your ability to manage infrastructure efficiently.

Embrace the power of for_each and start transforming how you deploy resources in your environment today. Happy coding!