Implement if/then/else syntax for conditional fields

Prerequisites

• I have read the documentation;
• In the case of a bug report, I understand that providing a SSCCE example is tremendously useful to the maintainers.

Description

The dependencies syntax is very powerful, however limitations are very quickly hit when dealing with fields across different objects. A usecase we have internally is: when: entity.legal_status != ‘limited_company’ then: ask for applicant.address

As additional incentive, there is a warning in the spec that dependencies could disappear.

Expected behavior

based on the example in the readme:

This can be written as chained if/then/else statements:

{
  "if": {"properties": {"Do you have any pets?": {"enum": ["No"]}}},
  "else": {
    "if": {"properties": {"Do you have any pets?": {"enum": ["Yes: One"]}}},
    "then": {
      "properties": {"How old is your pet?": {"type": "number"}},
      "required": ["How old is your pet?"]
    },
    "else": {
      "...": "etc"
    }
  }
}

or using a oneOf:

{
  "oneOf": [
    {
      "if": {"properties": {"Do you have any pets?": {"enum": ["No"]}}},
      "else": false
    },
    {
      "if": {"properties": {"Do you have any pets?": {"enum": ["Yes: One"]}}},
      "then": {
        "properties": {"How old is your pet?": {"type": "number"}},
        "required": ["How old is your pet?"]
      },
      "else": false
    },
    "..."
  ]
}

the false on else is to force the validity to evaluate falsely, so oneOf moves on

While implementing this, I came across a potential issue, in that utils.retrieveSchema doesn’t do a deep merge when resolving $refs. Given that the if/then/else syntax opens up the possibilities of doing cross-object conditioning, is this something that might need fixing? The main issue caused was that lists would not get appended, meaning if a $ref defined required, it would get discarded in favour of applied within a conditional.

If this is functionality people want here, I can prep a pull request.

[ultrox]

@cybojenix, sorry for intrusion, but is there any way you can share your fork, or just that part of the code where you implemented that.

[jay-hodgson]

PR #1610 is currently there as an implementation of this syntax.

@epicfaace : It looks like that PR has merge conflicts. Do those need to be resolved before consideration? (I don’t work in this project, so I apologize if this question is not applicable).