Equal of xsi:type in JSON Schema

Question

How can I hint the type of embedded objects in JSON Schema, analogous to xsi:type in XML Schema?

Example schema document:

{
    "type": "storeRequest",
    "properties": {
        "txid": {
            "description": "Transaction ID to prevent double committing",
            "type": "integer"
        },
        "objects": {
            "description": "Objects to store",
            "type": "array"
            "items": {
                "type": "object"
            },
            "minItems": 1,
            "uniqueItems": true
        },
    },
    "required": ["txid", "objects"]
}

This is a request the client sends to the server to store multiple objects in the database. Now how can I recursively validate the content of objects when it can contain more than one type of object. (Plymorphism, really).

I'd recommend you to use the OpenAPI support to polymorphism. I've written about it here: https://stackoverflow.com/questions/38626716/json-and-object-inheritance/46839490#46839490 — dbaltor, Feb 18 '20 at 08:10
@dbaltor Thanks for the good advice. How much support does this feature have in the different generated client/server stubs? — vbence, Feb 21 '20 at 12:54
Hi @vbence, haven't tried yet but it seems to support only client stubs as per this comment from the author: https://github.com/OpenAPITools/openapi-generator/pull/5120#issuecomment-588846548 — dbaltor, Mar 03 '20 at 12:22

score 1 · Answer 1 · answered Nov 11 '15 at 14:39

There is not an equivalent to xsi:type in JSON-schema AFAIK. Perhaps the most JSON-schema idiomatic way to hint the existence of types would be the explicit definition of types as schemas and referencing them through $ref:

{
    "properties" : {
        "wheels" : {
            "type" : "array",
            "items" : "$ref" : "#/definitions/wheel"
        }
    }

    "definitions" : {
        "wheel" : {
            "type" : "object"
        }
    }
}

Another way could be to give a hint through enums :

    {
    "definitions" : {
        "vehicle" : {
            "properties" : {
                "type" : {
                    "enum" : ["car", "bike", "plane"]
                }
            }
        },
        "plane" : {
            "properties" : {
                "type" : {
                    "enum" : "plane"
                }
            }
            "allOf" : ["$ref" : "#/definitions/vehicle"]
        }
     }
   }

Finally you can also add whatever tag you can process to a JSON-schema and follow your conventions.

Be aware that you are not going to find an equivalent translation between typical object oriented programming languages (java, C#) inheritance semantics and JSON-schema.

Thank you for the tips. What I was hoping for is some way to inject type information into the document which is being inspected. — vbence, Nov 11 '15 at 14:49
I would just an anyOf list of valid schemas for your objects, and use an enum to indicate in each actual object which schema it validates. — jruizaranguren, Nov 11 '15 at 15:16

Equal of xsi:type in JSON Schema

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