How to Get an Object from a List of Objects in Terraform?

TLDR

To retrieve an object from a list of objects in Terraform, use the for expression with filtering or the lookup function for direct access. This approach allows you to dynamically extract specific objects based on conditions.

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Terraform provides powerful tools for working with lists and objects. Retrieving a specific object from a list of objects is a common task in dynamic configurations. This guide explains how to achieve this with practical examples.

Step 1: Define a List of Objects

Start by defining a list of objects in your Terraform configuration.

Example

variable "instances" {
  default = [
    {
      name = "app-server"
      type = "t2.micro"
    },
    {
      name = "db-server"
      type = "t2.large"
    }
  ]
}

Explanation

• variable "instances": Declares a variable containing a list of objects.
• Each object has name and type attributes.

Step 2: Use a for Expression with Filtering

Use a for expression to filter the list and retrieve the desired object.

Example

locals {
  db_server = [for instance in var.instances : instance if instance.name == "db-server"]
}

Explanation

• for instance in var.instances: Iterates over the list of objects.
• if instance.name == "db-server": Filters objects based on the name attribute.
• db_server: Contains the filtered object(s).

Step 3: Extract a Single Object

If you expect only one object, use the tolist function to extract it.

Example

locals {
  db_server = tolist([for instance in var.instances : instance if instance.name == "db-server"])[0]
}

Explanation

• tolist: Converts the filtered result to a list.
• [0]: Extracts the first (and only) object from the list.

Step 4: Use the lookup Function

For direct access, use the lookup function if the list is indexed by a key.

Example

locals {
  instance_map = { for instance in var.instances : instance.name => instance }
  db_server    = lookup(local.instance_map, "db-server")
}

Explanation

• { for instance in var.instances : instance.name => instance }: Converts the list to a map indexed by name.
• lookup: Retrieves the object with the key db-server.

Best Practices

• Validate Inputs: Ensure the list contains the expected objects.
• Handle Missing Keys: Use the default argument in lookup to handle missing keys.
• Document Attributes: Clearly document the attributes of objects in the list.

By following these steps, you can efficiently retrieve objects from a list in Terraform, enabling dynamic and flexible configurations.