Patch validator must keep required with discriminator

[kapouer]

JSON Schema's OpenAPI specification allows some sort of schema inheritance:
https://ajv.js.org/json-schema.html#discriminator

To support this, objection's ajv "patch" validator should keep required field when it sits next a discriminator keyword.

To keep the change minimal, ajvNoDefaults.opts.discriminator is not checked to know if there is actual support for that keyword.

[kapouer]

NB this might help for issues like #1130.

[kapouer]

But this PR does not work with that case:

const Ajv = require("ajv")
const ajv = new Ajv({allErrors: true, strictSchema: true, discriminator: true, validateSchema: true})

const schema = {
  type: "object",
  properties: {
    foo: {type: "string"},
    bar: {type: "number", maximum: 3},
  },
  discriminator: { propertyName: 'test' },
  required: ["foo", "bar", "test"],
  oneOf: [{
    properties: {
        test: { const: 'one' },
        testa: { type: 'string' },
    },        
  }, {
    properties: {
        test: { const: 'two' },
        testb: { type: 'string' },
    },        
  },]
}

const validate = ajv.compile(schema)

test({foo: "abc", bar: 2, test: 'one', testa: 'toto'})
test({foo: 2, bar: 4, test: 'two', testb: 'tata'})

function test(data) {
  const valid = validate(data)
  if (valid) console.log("Valid!")
  else console.log("Invalid: " + ajv.errorsText(validate.errors))
}

It's actually possible to have shared properties + a discriminator.

[kapouer]

Force-pushed to fix it.

[lehni]

LGTM, thanks!

[kapouer]

Much appreciated

[lehni]

Released in v3.0.5