How to create a reusable block of statement for aws_iam_policy_document? - terraform0.12+

Question

I have a few aws_iam_policy_document like these
Policy1
data "aws_iam_policy_document" "policy1" {
statement {
actions = [
"rds:DescribeDBSnapshots"
]
resources = [
"arn:aws:rds:${var.region}:${local.account_id}:db:*
]
}
statement {
actions = [
"rds:RestoreDBInstanceFromDBSnapshot"
]
resources = [
"arn:aws:rds:${var.region}:${local.account_id}:db:*s",
"arn:aws:rds:${var.region}:${local.account_id}:snapshot:*"
]
}
}
Policy2
data "aws_iam_policy_document" "policy2" {
statement {
actions = [
"rds:DescribeDBInstances",
"rds:ModifyDBInstance"
]
resources = [
"arn:aws:rds:${var.region}:${local.account_id}:db:*",
"arn:aws:rds:${var.region}:${local.account_id}:db:*"
]
}
}
And I have a few more policies which all have some variations.
Now I want to be able insert an identical block of statement to all of these policies.
It is not tested yet but the statement will look like this:
statement {
action = [
"ssm:GetParameter"
]
resources = [
"arn:aws:ssm:${var.region}:${local.account_id}:parameter/lambda/*"
]
}
What is the most efficient way to do it? I want to avoid copy and paste the same block of statement to all policies.

Answer 1

I can use the source_json keyword.
Here is an example:
First of all, define the common policy in new policy document
data "aws_iam_policy_document" "common" {
statement {
action = [
"ssm:GetParameter"
]
resources = [
"arn:aws:ssm:${var.region}:${local.account_id}:parameter/lambda/*"
]
}
}
Then in the existing policy document, use source_json
data "aws_iam_policy_document" "policy1" {
source_json = "${data.aws_iam_policy_document.common.json}"
statement {
....
}
}