Json Schema Different required properties for first element in array? - jsonschema

Question

Suppose I want to validate an array in json where only the first element has an optional property. The rest of the items in the array would required the property.
schema.json
{
"type": "object",
"properties": {
"x": {
"type": "array",
"items": {
"oneOf": [
{ "$ref": "./first.json" },
{ "$ref": "./rest.json" }
]
}
}
}
}
first.json
{
"type": "object",
"properties": {
"y": { "type": "number" },
"z": { "type": "boolean" }
},
"required": ["y"]
}
rest.json
{
"type": "object",
"properties": {
"y": { "type": "number" },
"z": { "type": "boolean" }
}
}
Valid json:
{
"x": [
{ "z": true },
{ "y": 4, "z": true },
{ "y": 5 }
]
}
Invalid json:
{
"x": [
{ "z": true },
{ "z": true }, // this line would be invalid
{ "y": 5 }
]
}
However, the above setup gives me "multiple schemas match" when I provide the y value. Another approach could be:
schema.json
{
"type": "object",
"properties": {
"x": {
"type": "array",
"items": [
{ "$ref": "./first.json" },
{ "$ref": "./rest.json" }
]
}
}
}
This schema does what I want for the first two elements in the array (not requiring y in the first but yes in the second), but does not apply any schema to the objects after the second.

Answer 1

The items keyword has two forms. The first is the schema-form that you are using. The second is the array-form. The array-form takes an array of schemas where each schema applies to it's corresponding item in the instance. For example, when given an array with two schema, it validates the first item in the instance against the first schema and the second item against the second schema.
There is also an additionalItems keyword that works with the array-form of items. Any item that doesn't correspond to a schema in items is validated against the additionalItems schema. Using the previous example with two schemas in items, an instance with three items would have it's third item evaluated against the additionalItems schema.
So, we can use the array-form of items to apply "./first.json" to the first items and use additionalItems to apply "./rest.json" to the rest of the array.
{
"type": "object",
"properties": {
"x": {
"type": "array",
"items": [{ "$ref": "./first.json" }],
"additionalItems": { "$ref": "./rest.json" }
}
}
}