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Terraform

Terraform: Map List of Maps to Field Values List

By Filip on 11/17/2024

Learn how to effectively map a list of maps in Terraform to extract specific field values and create a new list tailored to your needs.

Terraform: Map List of Maps to Field Values List

Table of Contents

Introduction

In Terraform, working with lists of maps often involves extracting values associated with a specific key. This can be achieved efficiently using a combination of the for expression and the ... operator. Let's explore how to extract a list of values for a given key from a list of maps.

Step-by-Step Guide

To extract a list of values for a specific key from a list of maps in Terraform, you can use the for expression with the ... operator.

variable "list_of_maps" {
  default = [
    {"name" = "foo", "value" = "bar"},
    {"name" = "baz", "value" = "qux"}
  ]
}

output "values" {
  value = [for map in var.list_of_maps : map.value]
}

This code will iterate over each map in the list_of_maps variable and extract the value associated with the "value" key. The resulting values output will be a list containing ["bar", "qux"].

Here's a breakdown of the code:

  • for map in var.list_of_maps: This iterates over each map in the list_of_maps variable, assigning each map to the map variable.
  • map.value: This accesses the value associated with the "value" key in the current map.
  • [ ... ]: This constructs a new list containing the extracted values.

You can use this approach to extract values for any key from a list of maps. Simply replace "value" with the desired key name.

Code Example

This Terraform code defines a variable containing a list of maps, each with "name" and "value" keys. It then extracts the "value" and "name" values from each map into separate lists, outputting them as "values" and "names" respectively.

# Define a variable with a list of maps
variable "list_of_maps" {
  default = [
    {"name" = "foo", "value" = "bar"},
    {"name" = "baz", "value" = "qux"},
    {"name" = "quux", "value" = "corge"}
  ]
}

# Extract the "value" from each map and create a new list
output "values" {
  value = [for map in var.list_of_maps : map.value]
}

# Extract the "name" from each map and create a new list
output "names" {
  value = [for map in var.list_of_maps : map.name]
}

Explanation:

  1. variable "list_of_maps": This defines a variable named list_of_maps that holds a list of maps. Each map has two keys: "name" and "value".

  2. output "values": This defines an output variable named values.

  3. value = [for map in var.list_of_maps : map.value]: This is the core logic for extracting values. Let's break it down:

    • for map in var.list_of_maps: This iterates over each map within the list_of_maps variable. In each iteration, the current map is assigned to the temporary variable map.
    • map.value: This accesses the value associated with the key "value" within the current map.
    • [...]: The square brackets enclose the entire expression, indicating that the result should be a new list.

  4. output "names": This output variable is similar to output "values", but it extracts the values associated with the "name" key instead of "value".

Output:

When you run Terraform with this code, the outputs will be:

values = [
  "bar",
  "qux",
  "corge"
]

names = [
  "foo",
  "baz",
  "quux"
]

This demonstrates how to effectively extract specific key values from a list of maps in Terraform using the for expression and the ... operator.

Additional Notes

  • Real-world application: This pattern is particularly useful when working with data sources that return lists of maps, such as AWS instance metadata or outputs from modules. You can easily extract specific attributes from these complex data structures.
  • Error handling: If a key doesn't exist in a map, accessing it directly (e.g., map.nonexistent_key) will result in an error. To handle this gracefully, you can use the can function within the for expression: 
    output "optional_values" {
  value = [for map in var.list_of_maps : map.optional_key if can(map.optional_key)]
}
    This will only include values for maps that actually have the optional_key.
  • Alternative approaches: While the for expression with ... is generally the most readable and efficient way to extract values, you can also achieve similar results using functions like lookup within a loop. However, this can be less performant for larger lists.
  • Type constraints: For better code clarity and maintainability, consider adding type constraints to your variables and outputs. This helps ensure that the data being passed around matches your expectations.
  • Dynamic key access: You can even use a variable to dynamically specify the key you want to extract: 
    variable "key_to_extract" {
  default = "value"
}

output "dynamic_values" {
  value = [for map in var.list_of_maps : map[var.key_to_extract]]
}
    This adds flexibility to your code, allowing you to reuse the same logic for different keys.

Summary

This table summarizes how to extract a list of values for a specific key from a list of maps in Terraform:

Feature Description Example
Data Structure A list of maps, where each map represents a set of key-value pairs. [{"name" = "foo", "value" = "bar"}, {"name" = "baz", "value" = "qux"}]
Goal Extract the values associated with a specific key from all maps in the list. Get all values associated with the key "value".
Method Use a for expression with the spread operator (...) to iterate over the list and access the desired key in each map. [for map in var.list_of_maps : map.value]
Explanation - for map in var.list_of_maps: Iterates through each map in the list.
- map.value: Accesses the value associated with the "value" key in the current map.
- [ ... ]: Constructs a new list containing the extracted values.
Result A new list containing only the extracted values. ["bar", "qux"]
Adaptability Replace "value" with any desired key to extract values for that key.

Conclusion

Extracting specific values from lists of maps is a common task in Terraform, especially when dealing with complex data structures returned by providers or modules. By leveraging the for expression and the ... operator, you can efficiently create new lists containing only the desired values. Understanding this pattern empowers you to work with structured data effectively, enabling more dynamic and reusable Terraform configurations. Remember to handle optional keys gracefully and consider using type constraints for improved code clarity and maintainability. As you become more comfortable with these techniques, you'll be able to handle increasingly complex data manipulation scenarios in your infrastructure as code.

References

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