Terraform IAM policies combine

combinePolicies.tf

locals {
  policies = [ 
    # policies to combine in JSON
    # might be:
    # - data.aws_iam_policy.some-policy.policy
    # - aws_iam_policy.some-policy.policy
    # - file("some-policy.json")
    # - templatefile("some-policy.json.tpl", {})
    # - aws_iam_policy_document.some-policy.json
    aws_iam_policy.policy1.policy,
    templatefile("${path.module}/policies/some-policy.json.tpl", {
      aws_region = var.aws_region
      account_id = var.account_id
    })          
  ]
  combined = jsonencode({
    Statement = flatten(concat([
      for rules in [ 
        for policy in local.policies :
        jsondecode(policy).Statement
      ] :
      rules
    ]))
    Version = "2012-10-17" 
  })
}

# Usage
resource "aws_iam_policy" "example" {
  name = "my-policy"
  policy = local.combined
}