Simple flatten Terraform function example and it’s usage in various Terraform blocks.

Declare a local map

Local map variable with nested list of data.

locals {
  domains = {
    "example.com" = {
      enable = true,
      subdomains = [
        "mail.example.com",
        "email.example.com",
        "smtp.example.com"
      ]
    },
    "www.example.com" = {
      enable = false,
      subdomains = [
        "mail.www.example.com",
        "smtp.www.example.com"
      ]
    }
  }
}

output "original" {
  value = local.domains
}

Use flatten function

Using for loops, convert the local variable into a list of name value pairs.

output "flatmap" {
  value = flatten([
    for domain, obj in local.domains : [
      for subdomain in obj.subdomains : {
        zone      = domain
        subdomain = subdomain
        enabled = obj.enable
      }
    ]
  ])
}

Run terraform plan and you will see a flat map structure along with original map.

Terraform output

Changes to Outputs:
  + flatmap  = [
      + {
          + enabled   = true
          + subdomain = "mail.example.com"
          + zone      = "example.com"
        },
      + {
          + enabled   = true
          + subdomain = "email.example.com"
          + zone      = "example.com"
        },
      + {
          + enabled   = true
          + subdomain = "smtp.example.com"
          + zone      = "example.com"
        },
      + {
          + enabled   = false
          + subdomain = "mail.www.example.com"
          + zone      = "www.example.com"
        },
      + {
          + enabled   = false
          + subdomain = "smtp.www.example.com"
          + zone      = "www.example.com"
        },
    ]
  + original = {
      + "example.com"     = {
          + enable     = true
          + subdomains = [
              + "mail.example.com",
              + "email.example.com",
              + "smtp.example.com",
            ]
        }
      + "www.example.com" = {
          + enable     = false
          + subdomains = [
              + "mail.www.example.com",
              + "smtp.www.example.com",
            ]
        }
    }