How to extend an existing JSON schema?

[Hao HU]

Hi,

May I ask one naive question?

Suppose that there exists already a JSON schema. We have written some JSON codes on respecting this schema. Now we would like to extend this schema by adding some new properties. How would it be possible to extend the old schema in this way without breaking the support for old JSON codes?


Best regards,

Hao

[Hao HU]

Does there exist something like protobuf field number?

Regards,

Hao

[Geraint]

JSON data is generally tolerant of extra/unknown properties - they are usually just ignored.

Because of this, most schemas allow additional properties.  If this is the case, then you should be able to extend the schema using "allOf", and define some new properties:

{

    "allOf": [{"$ref": "/some/other/schema"}],

    "properties": {

        ...

    }

}

As long as the original schema did not specify "additionalProperties":false, then there should be no problem. :)

Geraint

[Hao HU]

Thanks a lot.

I heard that if we extend a schema with "additionalProperties":false specified, the new schema will not merge the properties of the parent schema into its own? Is that still true?

Or, If for example, I would like to overwrite a property of the parent schema. i.e delete one old property of the parent schema and then add a new one to take its place. How could it be possible in JSON schema? Or is there any alternatives to realize this?

Regards,

Hao

2013/11/13 Geraint <gerai...@gmail.com>

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[Geraint]

That is correct - if schema A bans additional properties, then a schema B extending that cannot define new properties.

If you want to extend the schema later, then you must not specify "additionalProperties":false.

Instead, if you really need to have no unknown properties (while allowing extensions), you could consider validating using "Ban Unknown Properties" mode.  A validator operating in this mode will disallow any properties that it does not recognise.  This mode is optional, but some validators already support it - what platform/environment are you using?

Geraint

[Hao HU]

Thanks,

I am sorry. I made a mistake. Actually what I want to say is that, I heard that even when we enable the "additionalProperties":true; the inherited properties won't be merged into the properties of child schema? You could see this link: https://groups.google.com/forum/#!topic/json-schema/Ru-tCCUlDUU

Regards,

Hao

[Geraint]

Ah, I see!

Yes - "additionalProperties" refers only to the properties defined in that immediate schema.

This allows you to restrict/ban properties in the sub-schema.  So if Schema A has some optional properties, then Schema B is allowed to extend Schema A but ban those properties - this is completely valid, because the properties are optional, so all data following Schema B will still follow Schema A.

If "additionalProperties" used properties from extended/referenced schemas, then this wouldn't be possible (as well as making implementations much more complex).

Does that make sense?

Geraint