JSON Schema: First Item of Array - arrays

Question

I need a JSON array of arbitrary length. Each item in the array is a JSON object, they all have same keys and types.
But the schema should make one exception: the first object doesn't need all keys, so schemas required list should be shorter for the first item.
I tried schemas with "items" and "prefixItems" without luck. It seems that "prefixItems" will be ignored independently of draft version when used with "items". Because array can be of arbitrary length, I guess I cannot use multiple schemas with "items".
{
"description": "Schema for array data",
"$schema": "http://json-schema.org/draft-2020-12/schema#",
"version": "0-0-6",
"type": "object",
"required": [
"data"
],
"properties": {
"data": {
"type": "array",
"prefixItems": [{
"type": "object",
"required": [
"name"
],
"properties": {
"name" : { "type" :"string" },
"age" : { "type" : "number" },
"city" : { "type" : "string" }
}
}],
"items":
{
"type": "object",
"required": [
"name", "age", "city"
],
"properties": {
"name" : { "type" :"string" },
"age" : { "type" : "number" },
"city" : { "type" : "string" }
}
}
}
}
}
My data:
{
"data" : [
{
"name": "Tom"
},
{
"name": "Ben",
"age": 32,
"city": "Berlin"
},
{
"name": "Mike",
"age": 40,
"city": "Boston"
}
]
}
For validation I use:
https://www.jsonschemavalidator.net/
https://jsonlint.com/
Validation of my example gives error for first item:
"Required properties are missing from object: age, city."

Answer 1

Use additionalItems instead of items.
Schema:
{
"$schema": "https://json-schema.org/draft/2020-12/schema",
"type": "array",
"prefixItems": [
{ "type": "number" },
{ "type": "string" },
{ "enum": ["Street", "Avenue", "Boulevard"] },
{ "enum": ["NW", "NE", "SW", "SE"] }
],
"additionalItems": {
"type": "number"
}
}
Data that validates:
[
1600,
"Pennsylvania",
"Avenue",
"NW",
1]
The issue you're running into is that the validators only support up to draft 7 of the standard, and using items the way we both want to was new in the 2019 draft. It's not supported yet.