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Terraform

Terraform: Get Object From List of Objects

By Filip on 10/07/2024

Learn how to efficiently extract specific objects from lists in Terraform using powerful built-in functions and simplify your infrastructure management.

Terraform: Get Object From List of Objects

Table of Contents

Introduction

In Terraform, locating a specific object within a list of objects is a common task. This article demonstrates how to achieve this using a combination of the for expression and the index function, allowing you to efficiently search and extract desired objects based on specific criteria.

Step-by-Step Guide

To find a specific object within a list of objects in Terraform, you can use a combination of the for expression and the index function.

Let's say you have a list of objects called object_list:

object_list = [
  {
    "id" = "abc1",
    "target" = "xyz1"
  },
  {
    "id" = "abc2",
    "target" = "xyz2"
  }
]

And you want to find the object where the id is "abc2". You can achieve this with the following code:

local {
  desired_object = [for obj in var.object_list : obj if obj.id == "abc2"][0]
}

Here's how it works:

  1. Iterate through the list: The for obj in var.object_list part iterates over each object (obj) in the object_list.

  2. Conditional Filtering: : obj if obj.id == "abc2" filters the objects, keeping only those where the id attribute matches "abc2".

  3. Extract the Object: The result of the for expression is a new list potentially containing the matching object. Since we expect only one match, we use [0] to extract the first element (our desired object) from the resulting list.

Now, the local.desired_object variable will hold the object with the id "abc2":

{
  "id" = "abc2",
  "target" = "xyz2"
}

You can then access the attributes of this object like this:

resource_id = local.desired_object.id
resource_target = local.desired_object.target

Code Example

This Terraform code defines a variable containing a list of objects. It then uses a local variable to filter the list and extract a specific object based on its "id" attribute. Finally, it defines output variables to display the extracted object and its "id" and "target" attributes.

variable "object_list" {
  default = [
    {
      "id" = "abc1",
      "target" = "xyz1"
    },
    {
      "id" = "abc2",
      "target" = "xyz2"
    }
  ]
}

locals {
  desired_object = [for obj in var.object_list : obj if obj.id == "abc2"][0]
  resource_id = local.desired_object.id
  resource_target = local.desired_object.target
}

output "desired_object" {
  value = local.desired_object
}

output "resource_id" {
  value = local.resource_id
}

output "resource_target" {
  value = local.resource_target
}

Explanation:

  1. Variable Definition: We define a variable object_list to hold the list of objects.
  2. Local Variable for Filtering: Inside the locals block, we define a variable desired_object.
  3. Filtering Logic: The for expression iterates through var.object_list. The if condition checks if the id attribute of the current object (obj) matches "abc2". If it does, the object is included in the resulting list.
  4. Extracting the Object: We use [0] to access the first element of the filtered list, which is our desired object.
  5. Accessing Attributes: We define additional local variables resource_id and resource_target to store the id and target attributes of the desired_object.
  6. Outputs: We define output variables to display the desired_object, resource_id, and resource_target.

Output:

desired_object = {
  "id" = "abc2",
  "target" = "xyz2"
}
resource_id = abc2
resource_target = xyz2

This code snippet demonstrates how to find a specific object within a list of objects in Terraform using for expression and index function. You can modify the id value to search for different objects within the list.

Additional Notes

  • Error Handling: The code assumes an object with the specified id exists. If not, it'll throw an error trying to access [0] on an empty list. Consider adding error handling using the length function and conditional logic to handle cases where no match is found.

  • Multiple Matches: If the list might contain multiple objects with the same id, using [0] will only return the first one. To handle multiple matches, you can skip the [0] indexing and iterate over the resulting list.

  • Alternative to index: While the example doesn't explicitly use the index function, it's useful for finding the index of an object within a list based on a condition. You could then use this index to access the specific object.

  • Performance: For very large lists, consider using alternative data structures like maps for more efficient lookups.

  • Real-world Applications: This pattern is valuable for tasks like:

    • Finding a specific security group by name.
    • Retrieving a subnet based on its availability zone.
    • Selecting a specific instance from a list based on tags.

  • Readability: Using meaningful variable names (like desired_object instead of just obj) significantly improves code readability.

Summary

This table summarizes how to find a specific object within a list of objects in Terraform:

Feature Description Example
Data Structure A list of objects, where each object has key-value pairs. object_list = [ { "id" = "abc1", "target" = "xyz1" }, { "id" = "abc2", "target" = "xyz2" } ]
Goal Find the object where a specific key matches a desired value. Find the object where "id" is "abc2".
Method Use a for expression with conditional filtering and index access. local { desired_object = [for obj in var.object_list : obj if obj.id == "abc2"][0] }
Explanation 1. Iterate: Loop through each object in the list using for obj in var.object_list.
2. Filter: Keep only the objects matching the condition obj.id == "abc2".
3. Extract: Access the first element (index [0]) of the filtered list, assuming a single match.
Result The desired_object variable will hold the matching object. { "id" = "abc2", "target" = "xyz2" }
Accessing Attributes Use dot notation to access the attributes of the found object. resource_id = local.desired_object.id
resource_target = local.desired_object.target

Conclusion

By combining the power of for expressions and the flexibility of conditional logic, Terraform enables you to efficiently search, filter, and extract specific objects from lists. This capability is essential for managing complex infrastructures where you often need to work with collections of resources and their properties. Understanding these techniques empowers you to write more dynamic and reusable Terraform code, ultimately leading to more maintainable and scalable infrastructure automation.

References

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