How to fix Error: The for_each value depends on resource attributes that cannot be determined until apply, so Terraform cannot predict how many instances will be created. in Terraform

# BAD - depends on resource ID (unknown at plan time)
for_each = toset(aws_instance.example[*].id)

# GOOD - uses static names from configuration
variable "instance_names" {
  type = list(string)
  default = ["web-1", "web-2", "web-3"]
}

for_each = toset(var.instance_names)

resource "aws_instance" "example" {
  count = 3
  ami   = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
}

terraform apply -target=aws_instance.example

terraform apply

# BAD - dynamic key
for_each = {
  for name, config in var.configs :
  aws_subnet.example[name].id => config  # ID unknown at plan time
}

# GOOD - dynamic value, static key
for_each = var.configs

resource "aws_route_table_association" "example" {
  for_each = each.value
  
  subnet_id      = aws_subnet.example[each.key].id  # Referenced in resource, not for_each
  route_table_id = aws_route_table.example.id
}

# BAD
module "monitoring" {
  for_each = toset(aws_instance.example[*].id)
  ...
}

# GOOD - pass instance names, derive dependencies in module
module "monitoring" {
  for_each = toset(var.instance_names)
  instance_id = aws_instance.example[each.value].id
}

# Instead of conditional iteration that fails in Terraform 1.6+
dynamic "rule" {
  for_each = var.enabled ? var.rules : []
  content {
    ...
  }
}

# Use tomap() to make it work
dynamic "rule" {
  for_each = var.enabled ? tomap({for r in var.rules : r.name => r}) : tomap({})
  content {
    ...
  }
}