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Mar 20, 2025
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Allow discriminator to be optional #4339

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f1d5629
First draft of optional discriminator feature
mikekistler Feb 6, 2025
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Address comments from PR review
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mikekistler Feb 11, 2025
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Use defaultMapping keyword in discriminator
mikekistler Feb 11, 2025
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Prefer Discriminator Object terminology
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Add back polymorphic example using allOf
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Update src/oas.md
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Use defaultMapping for unmapped discriminating values
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248 changes: 144 additions & 104 deletions src/oas.md
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Expand Up @@ -2832,7 +2832,7 @@ JSON Schema implementations MAY choose to treat keywords defined by the OpenAPI

| Field Name | Type | Description |
| ---- | :----: | ---- |
| <a name="schema-discriminator"></a>discriminator | [Discriminator Object](#discriminator-object) | Adds support for polymorphism. The discriminator is used to determine which of a set of schemas a payload is expected to satisfy. See [Composition and Inheritance](#composition-and-inheritance-polymorphism) for more details. |
| <a name="schema-discriminator"></a>discriminator | [Discriminator Object](#discriminator-object) | The discriminator provides a "hint" for which of a set of schemas a payload is expected to satisfy. See [Composition and Inheritance](#composition-and-inheritance-polymorphism) for more details. |
| <a name="schema-xml"></a>xml | [XML Object](#xml-object) | This MAY be used only on property schemas. It has no effect on root schemas. Adds additional metadata to describe the XML representation of this property. |
| <a name="schema-external-docs"></a>externalDocs | [External Documentation Object](#external-documentation-object) | Additional external documentation for this schema. |
| <a name="schema-example"></a>example | Any | A free-form field to include an example of an instance for this schema. To represent examples that cannot be naturally represented in JSON or YAML, a string value can be used to contain the example with escaping where necessary.<br><br>**Deprecated:** The `example` field has been deprecated in favor of the JSON Schema `examples` keyword. Use of `example` is discouraged, and later versions of this specification may remove it. |
Expand Down Expand Up @@ -2870,10 +2870,16 @@ The OpenAPI Specification allows combining and extending model definitions using
`allOf` takes an array of object definitions that are validated _independently_ but together compose a single object.

While composition offers model extensibility, it does not imply a hierarchy between the models.
To support polymorphism, the OpenAPI Specification adds the [`discriminator`](#schema-discriminator) field.
When used, the `discriminator` indicates the name of the property that hints which schema definition is expected to validate the structure of the model.
As such, the `discriminator` field MUST be a required field.
There are two ways to define the value of a discriminator for an inheriting instance.

JSON Schema also provides the `anyOf` and `oneOf` keywords, which allow defining multiple schemas where at least one or exactly one of them must be valid, respectively.
As is the case with `allOf`, the schemas are validated _independently_.
These keywords can be used to describe polymorphism, where a single field can accept multiple types of values.

The OpenAPI specification extends the JSON Schema support for polymorphism by adding the [`discriminator`](#schema-discriminator) field whose value is a [Discriminator Object](#discriminator-object).
When used, the Discriminator Object indicates the name of the property that hints which schema of an `anyOf` or `oneOf` is expected to validate the structure of the model.
The discriminating property may be defined as required or optional, but when defined as an optional property the Discriminator Object must include a `defaultMapping` field that specifies which schema of the `anyOf` or `oneOf` is expected to validate the structure of the model when the discriminating property is not present.

There are two ways to define the value of a discriminating property for an inheriting instance.

* Use the schema name.
* [Override the schema name](#discriminator-mapping) by overriding the property with a new value. If a new value exists, this takes precedence over the schema name.
Expand Down Expand Up @@ -3135,116 +3141,108 @@ components:

###### Models with Polymorphism Support

```json
{
"components": {
"schemas": {
"Pet": {
"type": "object",
"discriminator": {
"propertyName": "petType"
},
"properties": {
"name": {
"type": "string"
},
"petType": {
"type": "string"
}
},
"required": ["name", "petType"]
},
"Cat": {
"description": "A representation of a cat. Note that `Cat` will be used as the discriminating value.",
"allOf": [
{
"$ref": "#/components/schemas/Pet"
},
{
"type": "object",
"properties": {
"huntingSkill": {
"type": "string",
"description": "The measured skill for hunting",
"default": "lazy",
"enum": ["clueless", "lazy", "adventurous", "aggressive"]
}
},
"required": ["huntingSkill"]
}
]
},
"Dog": {
"description": "A representation of a dog. Note that `Dog` will be used as the discriminating value.",
"allOf": [
{
"$ref": "#/components/schemas/Pet"
},
{
"type": "object",
"properties": {
"packSize": {
"type": "integer",
"format": "int32",
"description": "the size of the pack the dog is from",
"default": 0,
"minimum": 0
}
},
"required": ["packSize"]
}
]
}
}
}
}
The following example describes a `Pet` model that can represent either a cat or a dog, as distinguished by the `petType` property. Each type of pet has other properties beyond those of the base `Pet` model. An instance without a `petType` property, or with a `petType` property that does not match either `cat` or `dog`, is invalid.

```yaml
components:
schemas:
Pet:
type: object
properties:
name:
type: string
required:
- name
- petType
oneOf:
- $ref: '#/components/schemas/Cat'
- $ref: '#/components/schemas/Dog'
Cat:
description: A pet cat
type: object
properties:
petType:
const: 'cat'
huntingSkill:
type: string
description: The measured skill for hunting
enum:
- clueless
- lazy
- adventurous
- aggressive
required:
- huntingSkill
Dog:
description: A pet dog
type: object
properties:
petType:
const: 'dog'
packSize:
type: integer
format: int32
description: the size of the pack the dog is from
default: 0
minimum: 0
required:
- packSize
```

###### Models with Polymorphism Support and a Discriminator Object

The following example extends the example of the previous section by adding a [Discriminator Object](#discriminator-object) to the `Pet` schema. Note that the Discriminator Object is only a hint to the consumer of the API, and does not change the validation outcome of the schema.

```yaml
components:
schemas:
Pet:
type: object
discriminator:
propertyName: petType
mapping:
cat: '#/components/schemas/Cat'
dog: '#/components/schemas/Dog'
properties:
name:
type: string
required:
- name
- petType
oneOf:
- $ref: '#/components/schemas/Cat'
- $ref: '#/components/schemas/Dog'
Cat:
description: A pet cat
type: object
properties:
petType:
const: 'cat'
huntingSkill:
type: string
description: The measured skill for hunting
enum:
- clueless
- lazy
- adventurous
- aggressive
required:
- huntingSkill
Dog:
description: A pet dog
type: object
properties:
petType:
const: 'dog'
packSize:
type: integer
format: int32
description: the size of the pack the dog is from
default: 0
minimum: 0
required:
- name
- petType
Cat: # "Cat" will be used as the discriminating value
description: A representation of a cat
allOf:
- $ref: '#/components/schemas/Pet'
- type: object
properties:
huntingSkill:
type: string
description: The measured skill for hunting
enum:
- clueless
- lazy
- adventurous
- aggressive
required:
- huntingSkill
Dog: # "Dog" will be used as the discriminating value
description: A representation of a dog
allOf:
- $ref: '#/components/schemas/Pet'
- type: object
properties:
packSize:
type: integer
format: int32
description: the size of the pack the dog is from
default: 0
minimum: 0
required:
- packSize
- packSize
```

###### Generic Data Structure Model
Expand Down Expand Up @@ -3362,7 +3360,9 @@ components:

#### Discriminator Object

When request bodies or response payloads may be one of a number of different schemas, a Discriminator Object gives a hint about the expected schema of the document.
When request bodies or response payloads may be one of a number of different schemas, these should use the JSON Schema `anyOf` or `oneOf` keywords to describe the possible schemas (see [Composition and Inheritance](#composition-and-inheritance-polymorphism)).

A polymorphic schema MAY include a Discriminator Object, which defines the name of the property that may be used as a hint for which schema of the `anyOf` or `oneOf` is expected to validate the structure of the model.
This hint can be used to aid in serialization, deserialization, and validation.
The Discriminator Object does this by implicitly or explicitly associating the possible values of a named property with alternative schemas.

Expand All @@ -3372,8 +3372,9 @@ Note that `discriminator` MUST NOT change the validation outcome of the schema.

| Field Name | Type | Description |
| ---- | :----: | ---- |
| <a name="property-name"></a>propertyName | `string` | **REQUIRED**. The name of the property in the payload that will hold the discriminating value. This property SHOULD be required in the payload schema, as the behavior when the property is absent is undefined. |
| <a name="property-name"></a>propertyName | `string` | **REQUIRED**. The name of the discriminating property in the payload that will hold the discriminating value. The discriminating property may be defined as required or optional, but when defined as optional the Discriminator Object must include a `defaultMapping` field that specifies which schema is expected to validate the structure of the model when no discriminating property is present. |
| <a name="discriminator-mapping"></a> mapping | Map[`string`, `string`] | An object to hold mappings between payload values and schema names or URI references. |
| <a name="default"></a> defaultMapping | `string` | The schema name or URI reference to a schema that is expected to validate the structure of the model when the discriminating property is not present in the payload. |
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The name defaultMapping suggests to me that this is the default whenever there isn't a defined mapping. So not just when the property is missing, but when the property has a value that cannot otherwise be resolved. Is this the intention? If not, I'd suggest a name tying to absence rather than "default."

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That was not the intention. But it is an interesting idea. This could be accomplished with a slightly different schema for the defaultMapping. Instead of not: required: ['petType'] it could be:

    properties:
      petType:
        not:
          enum: ['dog', 'cat']

This might get unwieldy for a schema with many variants, but there is some attractiveness to this.

Thoughts?


This object MAY be extended with [Specification Extensions](#specification-extensions).

Expand All @@ -3395,12 +3396,29 @@ The behavior of any configuration of `oneOf`, `anyOf`, `allOf` and `discriminato
The value of the property named in `propertyName` is used as the name of the associated schema under the [Components Object](#components-object), _unless_ a `mapping` is present for that value.
The `mapping` entry maps a specific property value to either a different schema component name, or to a schema identified by a URI.
When using implicit or explicit schema component names, inline `oneOf` or `anyOf` subschemas are not considered.
The behavior of a `mapping` value that is both a valid schema name and a valid relative URI reference is implementation-defined, but it is RECOMMENDED that it be treated as a schema name.
The behavior of a `mapping` value or `default` value that is both a valid schema name and a valid relative URI reference is implementation-defined, but it is RECOMMENDED that it be treated as a schema name.
To ensure that an ambiguous value (e.g. `"foo"`) is treated as a relative URI reference by all implementations, authors MUST prefix it with the `"."` path segment (e.g. `"./foo"`).

Mapping keys MUST be string values, but tooling MAY convert response values to strings for comparison.
However, the exact nature of such conversions are implementation-defined.

##### Optional discriminating property

When the discriminating property is defined as optional, the [Discriminator Object](#discriminator-object) must include a `defaultMapping` field that specifies a schema that is expected to validate the structure of the model when the discriminating property is not present in the payload. This allows the schema to still be validated correctly even if the discriminating property is missing.

The primary use case for an optional discriminating property is to allow a schema to be extended with a discriminator without breaking existing clients that do not provide the discriminating property.

Typically the schema specified in the `defaultMapping` field will specify that the discriminating property is not present, e.g.

```yaml
OtherPet:
type: object
not:
required: ['petType']
```

This will prevent the default mapping schema from validating a payload that includes the discriminating property, which would cause a validation of the payload to fail when polymorphism is described using the `oneOf` JSON schema keyword.

##### Examples

For these examples, assume all schemas are in the [entry document](#openapi-description-structure) of the OAD; for handling of `discriminator` in referenced documents see [Resolving Implicit Connections](#resolving-implicit-connections).
Expand All @@ -3415,7 +3433,7 @@ MyResponseType:
- $ref: '#/components/schemas/Lizard'
```

which means the payload _MUST_, by validation, match exactly one of the schemas described by `Cat`, `Dog`, or `Lizard`. Deserialization of a `oneOf` can be a costly operation, as it requires determining which schema matches the payload and thus should be used in deserialization. This problem also exists for `anyOf` schemas. A `discriminator` MAY be used as a "hint" to improve the efficiency of selection of the matching schema. The `discriminator` field cannot change the validation result of the `oneOf`, it can only help make the deserialization more efficient and provide better error messaging. We can specify the exact field that tells us which schema is expected to match the instance:
which means the payload _MUST_, by validation, match exactly one of the schemas described by `Cat`, `Dog`, or `Lizard`. Deserialization of a `oneOf` can be a costly operation, as it requires determining which schema matches the payload and thus should be used in deserialization. This problem also exists for `anyOf` schemas. A `discriminator` MAY be used as a "hint" to improve the efficiency of selection of the matching schema. The Discriminator Object cannot change the validation result of the `oneOf`, it can only help make the deserialization more efficient and provide better error messaging. We can specify the exact field that tells us which schema is expected to match the instance:

```yaml
MyResponseType:
Expand All @@ -3438,7 +3456,7 @@ The expectation now is that a property with name `petType` _MUST_ be present in

will indicate that the `Cat` schema is expected to match this payload.

In scenarios where the value of the `discriminator` field does not match the schema name or implicit mapping is not possible, an optional `mapping` definition MAY be used:
In scenarios where the value of the discriminating property does not match the schema name or implicit mapping is not possible, an optional `mapping` definition MAY be used:

```yaml
MyResponseType:
Expand All @@ -3458,6 +3476,28 @@ Here the discriminating value of `dog` will map to the schema `#/components/sche

When used in conjunction with the `anyOf` construct, the use of the discriminator can avoid ambiguity for serializers/deserializers where multiple schemas may satisfy a single payload.

When the discriminating property is defined as optional, the Discriminator Object must include a `defaultMapping` field that specifies a schema of the `anyOf` or `oneOf` is expected to validate the structure of the model when the discriminating property is not present in the payload. This allows the schema to still be validated correctly even if the discriminator property is missing.

For example:

```yaml
MyResponseType:
oneOf:
- $ref: '#/components/schemas/Cat'
- $ref: '#/components/schemas/Dog'
- $ref: '#/components/schemas/Lizard'
- $ref: '#/components/schemas/OtherPet'
discriminator:
propertyName: petType
defaultMapping: OtherPet
OtherPet:
type: object
not:
required: ['petType']
```

In this example, if the `petType` property is not present in the payload, the payload should validate against the `OtherPet` schema.

This example shows the `allOf` usage, which avoids needing to reference all child schemas in the parent:

```yaml
Expand Down