How to validate sum of refs in Joi

Question

How can I validate the total of two refs as below? I would prefer have custom be on the total, yet unsure of how to get the value of the refs in the custom when using Joi.ref.

I can do the following and hang a custom off the entire validation schema, but would prefer to attach it to total.

I don't want to use expression()

I prefer not to change the schema structure as per this answer.

  const widgetValidator = Joi.object({
    a: Joi.number().integer().min(0).required(),
    b: Joi.number().integer().min(1).required(),
    total: Joi.number().integer().min(1).required(),
  }).custom((value: {a: number; b: number; total: number;}, helpers) => {
      if (value.total !== value.a + value.b) {
        throw new Error('invalid balance calculation');
      }

      return value;
    },
  );

Answer 1

When you install custom on an attribute, it will be a little hard to get whole object values. You have to get it from helpers's states:

Joi.object({
    a: Joi.number().integer().min(0).required(),
    b: Joi.number().integer().min(1).required(),
    total: Joi.number().integer().min(1).required().custom((value, helpers) => {
      const { a, b } = helpers.state.ancestors[0];
      if (value !== a + b) {
        throw new Error('invalid balance calculation');
      }

      return value;
    },
  )
})