🐛 Support custom OpenAPI / JSON Schema fields in the generated output OpenAPI

[jacobsvante]

Pydantic supports adding custom fields other than example and examples using BaseModel.Config.schema_extra. This change ensures that those extra fields exist in the FastAPI generated Open API schema as well.

This is in line with what the current docs say:

Other info
The same way, you could add your own custom extra info that would be added to the JSON Schema for each model, for example to customize a frontend user interface, etc.

(https://fastapi.tiangolo.com/tutorial/schema-extra-example/#other-info)

[codecov]

Codecov Report

Merging #1429 (0868a37) into master (79f52c7) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1429   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          248       249    +1     
  Lines         7717      7740   +23     
=========================================
+ Hits          7717      7740   +23     

Impacted Files	Coverage Δ
fastapi/openapi/models.py	100.00% <100.00%> (ø)
tests/test_custom_schema_fields.py	100.00% <100.00%> (ø)

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[github-actions]

📝 Docs preview for commit 3c20f23 at: https://60e098eaae1c69f28f225bd6--fastapi.netlify.app

[tiangolo]

Good point, thanks @jmagnusson! Thanks for your contribution! ☕

And thanks for the patience 😅

[johnthagen]

Are custom fields supported by the OpenAPI spec?

I saw breakages using OpenAPI generator's schema validation when custom fields started appearing in the generated schema starting in FastAPI 0.66.0, which includes this PR.

A minimal reproducible example here: ormar-orm/ormar#291

[jacobsvante]

Are custom fields supported by the OpenAPI spec?

Yes, see Specification Extensions here: https://swagger.io/specification/. All x- prefixed keys are allowed (though they mentioned that not all tooling supports it.)

[ml-evs]

I saw breakages using OpenAPI generator's schema validation when custom fields started appearing in the generated schema starting in FastAPI 0.66.0, which includes this PR.

We ran into this when just using some pydantic Field arguments (e.g. const) that now get erroneously added to the OpenAPI schema. I raised #3745 on this if it needs to be discussed; if I can get some consensus that it is a bug then I am happy to try and fix it.