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docs: FAQ about resolving $ref in schemas

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Evgeny Poberezkin 2017-04-06 06:50:58 +01:00
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@ -57,3 +57,25 @@ There are several ways to implement the described logic that would allow two pro
This problem is related to the problem explained above - properties treated as additional in the sence of `additionalProperties` keyword, based on `properties`/`patternProperties` keyword in the same schema object.
See the exemple in [Filtering Data](https://github.com/epoberezkin/ajv#filtering-data) section of readme.
## Generating schemas with resolved references ($ref)
See [#22](https://github.com/epoberezkin/ajv/issues/22), [#125](https://github.com/epoberezkin/ajv/issues/125), [#146](https://github.com/epoberezkin/ajv/issues/146), [#228](https://github.com/epoberezkin/ajv/issues/228), [#336](https://github.com/epoberezkin/ajv/issues/336)
##### Why Ajv does not replace references ($refs) with the actual referenced schemas as some validators do?
1. The scope of Ajv is validating data against JSON-Schemas; inlining referenced schemas is not necessary for validation. When Ajv generates code for validation it either inlines the code of referenced schema or uses function calls. Doing schema manipulation is more complex and out of scope.
2. When schemas are recursive (or mutually recursive) resolving references would result in recursive data-structures, that can be difficult to process.
3. There are cases when such inlining would also require adding (or modyfing) `id` attribute in the inlined schema fragment to make the resulting schema equivalent.
There were many conversations about the meaning of `$ref` in [JSON-Schema GitHub organisation](https://github.com/json-schema-org). The consesus is that while it is possible to treat `$ref` as schema inclusion with two caveats (above), this interpretation is unnecessary complex. A more efficient approach is to treat `$ref` as delegation, i.e. a special keyword that validates the current data instance against the referenced schema. The analogy with programming languages is be that `$ref` is a function call rather than a macro. See [here](https://github.com/json-schema-org/json-schema-spec/issues/279), for example.
##### How can I generate a schema where `$ref`s are replaced with referenced schemas?
There are two possible approaches:
1. Write code to traverse schema and replace every `$ref` with the referenced schema. An additional limitation is that `"$ref"`s inside keywords "properties", "patternProperties" and "dependencies" mean property name (or pattern) rather than the reference to another schema.
2. Use a specially constructed JSON Schema with a [custom keyword](https://github.com/epoberezkin/ajv/blob/master/CUSTOM.md) to traverse and modify your schema.