JSON schema which allows either an object or an array of those objects

Question

Say I have a JSON schema that allows for an object like so:

...
  "assetMetadata": {
    "type": "object",
    "additionalProperties": false,
    "properties": { ... }
  }
...

So say I want to change this to allow either that same object OR an array of that particular object. Here is accepting just an array:

...
"assetMetadata": {
  "type": "array",
  "description": "...",
  "items": {
    "type": "object",
    "additionalProperties": false,
    "properties": {...}
}
...

The properties are the same (it's the same object, just the option for multiple instead of just one).

Interestingly enough in the project I'm working on, the unmarshaller can already handle both (it turns the single object into a sequence of size 1), so it's purely the validation that's preventing me from going forward. We want to maintain comparability with the existing API, which is the reason I can't just require an array now.

Answer 1

You can achieve this using the anyOf keyword and definitions/$ref to avoid duplication.

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "object",
  "properties": {
    "assetMetadata": {
      "anyOf": [
        { "$ref": "#/definitions/assetMetaData" },
        {
          "type": "array",
          "description": "...",
          "items": { "$ref": "#/definitions/assetMetaData" }
        }
      ]
    }
  },
  "definitions": {
    "assetMetadata": {
      "type": "object",
      "additionalProperties": false,
      "properties": { ... }
    }
  }
}