I have JSON schema file where one of the properties is defined as either string or null:
"type":["string", "null"]
When converted to YAML (for use with OpenAPI/Swagger), it becomes:
type:
- 'null'
- string
but the Swagger Editor shows an error:
Schema "type" key must be a string
What is the correct way to define a nullable property in OpenAPI?
This depends on the OpenAPI version.
Your example is valid in OpenAPI 3.1 and later versions, which are fully compatible with JSON Schema 2020-12.
type:
- 'null' # Note the quotes around 'null'
- string
# same as
type: ['null', string]
The above is equivalent to:
oneOf:
- type: 'null' # Note the quotes around 'null'
- type: string
The nullable keyword used in OAS 3.0.x (see below) does not exist in OAS 3.1+, it was removed in favor of the 'null' type.
Nullable strings are defined as follows:
type: string
nullable: true
This is different from JSON Schema syntax because OpenAPI versions up to 3.0.x use their own flavor of JSON Schema ("extended subset"). One of the differences is that the type must be a single type and cannot be a list of types. Also there's no 'null' type; instead, the nullable keyword serves as a type modifier to allow null values.
Version 2 does not support 'null' as the data type, so you are out of luck. You can only use type: string. But some tools support the vendor extension x-nullable: true to indicate nullable properties.
Consider migrating to OpenAPI v. 3 to get proper support for nulls.
Multiple types via an array are not supported
3.0.3 specifically, nullable: true actually allows the type: 'nullable' value to be used. So you want to do nullable: true and type: 'null' together. spec.openapis.org/oas/v3.0.3#fixed-fields-19
type: 'null' keyword in OpenAPI 3.0.x; instead you're supposed to use nullable: true as explained in this answer. The wording in the spec section you linked to is a bit confusing, but the intended meaning is that the value null is considered a valid value for a schema instance if this schema has the nullable: true attribute.type: 'null' and type: [....., 'null'] are valid only is OpenAPI 3.1.