Context

• node version: 9.11.1
• joi version:13.1.2
• environment (node, browser):node
• used with (hapi, standalone, ...):standalone
• any other relevant information:

What are you trying to achieve or the steps to reproduce ?

let Joi = require('joi');

const objSchema = Joi.object({
   foo: Joi.string().optional()
});

const arrSchema = Joi.array().items(objSchema).unique('foo');

let validationResult = arrSchema.validate([{}, {}]);

if (!validationResult.error) {
    console.log('OK!');
} else {
    let detail = validationResult.error.details[0];
    if (detail.type === 'array.unique') {
        // Create a user-friendly custom error message
        let context = detail.context;
        console.error(`Duplicate element ${context.path} for objects at indices ${context.pos} and ${context.dupePos}`);
    } else {
        // Just log the Joi error
        console.error(detail.message);
    }
}

Which result you had ?

output was:

Duplicate element foo for objects at indices 1 and 0

That is, I got a validation error because the two objects were missing the optional string element 'foo'. I understand that Joi has interpreted this as if I was validating [{foo:undefined}, {foo:undefined}], for which clearly the value of foo is not unique among the array elements. This is documented behaviour.

What did you expect ?

1. I'd like to not fail the validation when these optional fields are undefined... ie only check uniqueness among the defined fields 'foo'.
2. Additonally, I'd like to construct a custom error message by using the error detail context path.

I can acheive 1 by doing:

const arrSchema = Joi.array().items(objSchema).unique((a,b) => a.foo && b.foo && a.foo === b.foo);

but then there's no path element defined for me to construct an error message with.

I wonder if something like the following makes sense:

• allow an additional param to unique('foo') to compare only defined values. Or,
• allow an additional param to unique((a,b) => a.foo && b.foo && a.foo === b.foo) to provide the path in the error.

Any thoughts?

Thank you.