Compose schemas #46

[aboutlo]

Normally inheritance with JSON Schema is achieved with allOf. However when .additionalProperties(false) is used the validator won't
understand which properties come from the base schema. S.extend creates a schema merging the base into the new one so that the validator knows all the properties because it's evaluating only a single schema.
For example in a CRUD API POST /users could use the userBaseSchema rather than GET /users or PATCH /users use the userSchema whose contains the id, createdAt and updatedAt generated server side

related to #46

[mcollina]

Good work! Do you think it could be possible for the inherited schema to be already a JSON schema and not our wrapper?

[aboutlo]

Thank you @mcollina!

The problem is that our in-memory data structure is different.
I use an array of properties so I know what is the last property and I can add there the attributes.
https://github.com/fastify/fluent-schema/blob/master/src/utils.js#L140-L150

Later I deserialize:
https://github.com/fastify/fluent-schema/blob/master/src/BaseSchema.js#L372

So if we want to accept a JSON schema I guess I need to parse it. On the other hand, I'm starting to see that having something like S.raw() could be interesting and it could be applied here as well.

[mcollina]

LGTM

[mcollina]

This is ok as-is, I would move the discussion on S.raw() in another issue.