DateTime(format="timestamp") generates invalid OpenAPI schema using min instead of minimum

[ULisichkin]

Description

apispec generates an invalid OpenAPI schema for marshmallow.fields.DateTime(format="timestamp").

The generated schema contains the min property:

changedOn:
  type: number
  format: float
  example: "1676451245.596"
  min: "0"

However, min is not a valid OpenAPI Schema Object property. As a result, the generated specification fails validation with openapi-spec-validator:

OpenAPIValidationError:
Property 'min' is not allowed

This was reproduced with:

• apispec 6.10.0
• marshmallow 3.26.2
• openapi-spec-validator 0.9.0
• OpenAPI 3.0.2

Expected behavior

The generated schema should use the standard OpenAPI keyword minimum instead of min, for example:

changedOn:
  type: number
  format: float
  example: "1676451245.596"
  minimum: 0

or omit the constraint entirely if no minimum value is intended.

Minimal reproduction

from apispec import APISpec
from apispec.ext.marshmallow import MarshmallowPlugin
from marshmallow import Schema, fields
from pprint import pprint

class TestSchema(Schema):
    ts = fields.DateTime(format="timestamp")

spec = APISpec(
    title="test",
    version="1.0",
    openapi_version="3.0.2",
    plugins=[MarshmallowPlugin()],
)

spec.components.schema("Test", schema=TestSchema)

pprint(spec.to_dict()["components"]["schemas"]["Test"])

Output:

{'additionalProperties': False,
 'properties': {'ts': {'example': '1676451245.596',
                       'format': 'float',
                       'min': '0',
                       'type': 'number'}},
 'type': 'object'}

The file field_converter.py contains the code that causes this issue:

            elif field.format == "timestamp":
                ret = {
                    "type": "number",
                    "format": "float",
                    "example": "1676451245.596",
                    "min": "0",
                }
            elif field.format == "timestamp_ms":
                ret = {
                    "type": "number",
                    "format": "float",
                    "example": "1676451277514.654",
                    "min": "0",
                }

Specification reference

The OpenAPI Schema Object defines the numeric constraint keyword as minimum; min is not a valid keyword.

OpenAPI 3.0.2 Schema Object:
https://spec.openapis.org/oas/v3.0.2#schema-object