oneOf validate failed

What version of Ajv are you using? Does the issue happen if you use the latest version? 8.10.0

Ajv options object

{}

JSON Schema

{
  "title": "Block object",
  "type": "object",
  "required": [
    "transactions"
  ],
  "properties": {
    "transactions": {
      "oneOf": [
        {
          "title": "Full transactions",
          "type": "array",
          "items": {
            "title": "Signed 1559 Transaction",
            "type": "object",
            "required": [
              "chainId",
              "input"
            ],
            "properties": {
              "input": {
                "title": "input data",
                "type": "string",
                "pattern": "^0x[0-9a-f]*$"
              },
              "chainId": {
                "title": "chainId",
                "type": "string",
                "pattern": "^0x([1-9a-f]+[0-9a-f]*|0)$",
                "description": "Chain ID that this transaction is valid on."
              }
            }
          }
        },
        {
          "title": "Transaction hashes",
          "type": "array",
          "items": {
            "title": "32 byte hex value",
            "type": "string",
            "pattern": "^0x[0-9a-f]{64}$"
          }
        }
      ]
    }
  }
}

Sample data

{
  "transactions": []
}

Your code

const ajv = new Ajv();
const blockSchemaValidator = ajv.compile(schema);
const valid = blockSchemaValidator(block.result);

Validation result, data AFTER validation, error messages

// valid: false
[
  {
    instancePath: '/transactions',
    schemaPath: '#/properties/transactions/oneOf',
    keyword: 'oneOf',
    params: { passingSchemas: [Array] },
    message: 'must match exactly one schema in oneOf'
  }
]

What results did you expect? The result should pass , because the transactions field is an empty array

Are you going to resolve the issue? Not Sure

[epoberezkin]

@JacobLey thank you!

[JacobLey]

It would be easier to review with a more minimal schema, but on my review it appears that this is an expected behavior.

Both oneOf schemas are an array, just with different items. But neither specifies a minItems or something like that to disqualify an empty array. So your example data with an empty array validates against both, hence failing validation for oneOf.

There are a couple ways to go about this, but I’d recommend just adding a minItems: 1 to one of your array schemas. That will disqualify it from validation, and allow the other to pass as the only valid schema in oneOf.

Alternatively could switch to anyOf instead of oneOf. The only case where both are valid is empty should be an empty array, which sounds fine? I suspect that is a slight performance gain as well, because AJV internals can skip the second schema if the first is valid

So it looks like a bug in your schema, not AJV