addSchema() produces error when {nullable: true}

[maldimirov]

Using nullable: true option when calling addSchema() an error is thrown:

Error: schema with key or id "" already exists

What version of Ajv are you using?
Ajv version: 6.12.0 (atm latest)

Example code to reproduce

const Ajv = require('ajv');

const schema = {
    type: 'string',
    nullable: true,
};
const value1 = 'string';
const value2 = null;

const ajv = new Ajv({nullable: true});
ajv.addSchema(schema, '#');
const valid1 = ajv.validate('#', value1);
const valid2 = ajv.validate('#', value2);

console.log(valid1);
console.log(valid2);

A bit more details
On the example code when you remove the nullable option everything works as expected and the console prints true and false. Setting the option to either nullable: true or nullable: false produces the error.

What results did you expect?
Not throwing error and console logging true 2 times.

Are you going to resolve the issue?
No, but I will provide a workaround in the comments.

[maldimirov]

Workaround
A workaround is using compile(). E.g.

const ajv = new Ajv({nullable: true});
const validate = ajv.compile(schema);
const valid1 = validate(value1);
const valid2 = validate(value2);

So why use nullable and addSchema()?
We are using Ajv to validate API responses against an Open Api Schema definition and it's really convenient to do something like:

const ajv = new Ajv({nullable: true});
ajv.addSchema(apiDoc, '#')
  .validate('#/components/schemas/Details', response.body)

In this case the workaround we found is to "trick" compile() using a $ref like this:

const validate = ajv.compile({
   $ref: '#/components/schemas/Details',
   ...schema,
});
validate(response.body);

[epoberezkin]

A better approach would be to give schema some ID and then using getSchema or validate:

ajv.addSchema(schema, 'foo')
validate = ajv.getSchema('foo#/components/schemas/Details')
validate(data)

// or
ajv.validate('foo#/components/schemas/Details', data)

[maldimirov]

I thought a # is a valid schema name. Guess not :)
Your example works, but it would be good to have that documented somewhere. I could not find in the documentation any reference on what is a valid schema name, and that passing # will actually result in a schema without id.

[epoberezkin]

schema id is URI with JSON pointer in hash fragment, so “foo” and “foo#” are treated as the same schema id. That’s why # on its own is an empty URI.

I am still not sure why it works without nullable and doesn’t work with it...

[maldimirov]

Well, I leave it to you to decide whether this is a bug and the issue should remain open.

[epoberezkin]

Yes, I want to investigate it at some point. It is definitely worth clarifying schema ID in docs, in line with the specification, in this section: https://github.com/epoberezkin/ajv#ref

All the examples there have full URI as schema IDs.

[jasoniangreen]

This works in the latest version.