Dynamic block with optional data - dynamic

Question

let say we have this locals:
locals = {
schemas = [
{
name = "is_cool"
attribute_data_type = "Boolean"
mutable = true
required = false
},
{
name = "firstname"
attribute_data_type = "String"
mutable = true
required = false
min_length = 1
max_length = 256
}
]
}
What I would like to achieve is to use dynamic to build schemas and when the schema is a string I would like to add the string_attribute_constraints block.
This is what I did so far but it adds an empty string_attribute_constraints block when the schema is Boolean
dynamic "schema" {
for_each = var.schemas
content {
name = schema.value.name
attribute_data_type = schema.value.attribute_data_type
mutable = schema.value.mutable
required = schema.value.required
string_attribute_constraints {
min_length = lookup(schema.value, "min_length", null)
max_length = lookup(schema.value, "max_length", null)
}
}
}
terraform plan:
+ schema {
+ attribute_data_type = "Boolean"
+ mutable = true
+ name = "is_cool"
+ required = false
+ string_attribute_constraints {}
}

Answer 1

You can use a second nested dynamic block to tell Terraform how many string_attribute_constraints blocks to generate based on your rule:
dynamic "schema" {
for_each = var.schemas
content {
name = schema.value.name
attribute_data_type = schema.value.attribute_data_type
mutable = schema.value.mutable
required = schema.value.required
dynamic "string_attribute_constraints" {
for_each = schema.attribute_data_type == "String" ? [1] : []
content {
min_length = lookup(schema.value, "min_length", null)
max_length = lookup(schema.value, "max_length", null)
}
}
}
}
This works by making the for_each for the nested dynamic be an empty list in the case where we don't want to generate any blocks, and making it a single-element list in the case where we do. Since we need no references to string_attribute_constraints.key or string_attribute_constraints.value inside the block, we can set the value of the single element to anything, and so I just set it to 1 as an arbitrary placeholder.

Answer 2

dynamic "schema" {
for_each = local.my_schema
content {
name = schema.value.name
attribute_data_type = schema.value.attribute_data_type
mutable = schema.value.mutable
required = schema.value.required
dynamic "string_attribute_constraints" {
for_each = schema.value.attribute_data_type == "String" ? [1] : []
content {
min_length = lookup(schema.value, "min_length", 0)
max_length = lookup(schema.value, "max_length", 2048)
}
}
dynamic "number_attribute_constraints" {
for_each = schema.value.attribute_data_type == "Number" ? [1] : []
content {
min_value = lookup(schema.value, "min_value", 0)
max_value = lookup(schema.value, "max_value", 2048)
}
}
}
}