ajv-merge-patch
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$merge and $patch keywords for Ajv JSON-Schema validator to extend schemas
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# ajv-merge-patch
$merge and $patch keywords for [Ajv JSON-Schema validator](https://github.com/epoberezkin/ajv) to extend JSON-schemas
[](https://github.com/ajv-validator/ajv-merge-patch/actions/workflows/build.yml)
[](https://www.npmjs.com/package/ajv-merge-patch)
[](https://coveralls.io/github/epoberezkin/ajv-merge-patch?branch=master)
## Problem solved
The keywords `$merge` and `$patch` allow to extend the JSON-schemas using patches in the format [JSON Merge Patch (RFC 7396)](https://tools.ietf.org/html/rfc7396) or [JSON Patch (RFC 6902)](https://tools.ietf.org/html/rfc6902).
Schema extension is necessary if you want to add additional properties to the recursive schema (e.g. meta-schema). Consider this example:
Original schema:
```json
{
"id": "mySchema.json#",
"type": "object",
"properties": {
"foo": { "type": "string" },
"bar": { "$ref": "#" }
},
"additionalProperties": false
}
```
Valid data: `{ foo: 'a' }`, `{ foo: 'a', bar: { foo: 'b' } }` etc.
If you want to define schema that would allow more properties, the only way to do it without `$merge` or `$patch` keywords is to copy-paste and edit the original schema.
Using `$merge` keyword you can create an extended schema in this way:
```json
{
"id": "mySchemaExtended.json#",
"$merge": {
"source": { "$ref": "mySchema.json#" },
"with": {
"properties": {
"baz": { "type": "number" }
}
}
}
}
```
Valid data: `{ foo: 'a', baz: 1 }`, `{ foo: 'a', baz: 1, bar: { foo: 'b', baz: 2 } }`, etc.
`$merge` is implemented as a [custom macro keyword](https://github.com/epoberezkin/ajv/blob/master/CUSTOM.md#define-keyword-with-macro-function) using [json-merge-patch](https://github.com/pierreinglebert/json-merge-patch) package.
The same schema extension using `$patch` keyword:
```json
{
"id": "mySchemaExtended.json#",
"$patch": {
"source": { "$ref": "mySchema.json#" },
"with": [
{
"op": "add",
"path": "/properties/baz",
"value": { "type": "number" }
}
]
}
}
```
`$patch` is implemented as a [custom macro keyword](https://github.com/epoberezkin/ajv/blob/master/CUSTOM.md#define-keyword-with-macro-function) using [fast-json-patch](https://github.com/Starcounter-Jack/JSON-Patch) package.
In the majority of cases `$merge` format is easier to understand and to maintain. `$patch` can be used for extensions and changes that cannot be expressed using `$merge`, e.g. [Adding an array value](https://tools.ietf.org/html/rfc6902#page-18).
`with` property in keywords can also be a reference to a part of some schema, in which case the resolved value will be used rather than the actual object with property `$ref`.
__Please note__:
1. In case the `source` schema or the patch in `with` keyword use `$ref`, they won't take into account the resolution scope for their internal $refs defined by the parent objects - they will be used as separate objects.
2. `$ref` in both `source` schema and `with` patch take into account current $ref resolution scope (from version 2.0.0).
See also:
- [v5 proposal](https://github.com/daveclayton/json-schema-validator/wiki/v5:-merge)
- [$merge and $patch tests](https://github.com/epoberezkin/ajv-merge-patch/blob/master/spec)
- [discussion in JSON-Schema-Org](https://github.com/json-schema-org/json-schema-spec/issues/15)
## Usage with Ajv
These keywords are compatible with Ajv version >=5.1.0-beta.0.
To add these keywords to Ajv instance:
```javascript
var Ajv = require('ajv');
var ajv = new Ajv();
require('ajv-merge-patch')(ajv);
```
## Using in the browser
You can include these keywords in your code using browserify.
To include only `$merge` keyword:
```javascript
require('ajv-merge-patch/keywords/merge')(ajv);
```
To include only `$patch` keyword:
```javascript
require('ajv-merge-patch/keywords/patch')(ajv);
```
## License
[MIT](https://github.com/epoberezkin/ajv-merge-patch/blob/master/LICENSE)