Tutorial [EN] speak about ordering in Union variants

Describe the bug

The example in /fastapi/blob/master/docs_src/extra_models/tutorial003.py describe how to use a Union in a response_model like so:

@app.get("/items/{item_id}", response_model=Union[PlaneItem, CarItem])
async def read_item(item_id: str):
    return items[item_id]

However this will not work as expected if we will to flip the order of the items inside the Union[...].

@app.get("/items/{item_id}", response_model=Union[CarItem, PlaneItem])
async def read_item(item_id: str):
    return items[item_id]

In that case the "size": 5 will not be returned for "item2".

And there is no mention of the issue anywhere in the text at:

https://fastapi.tiangolo.com/tutorial/extra-models/#union-or-anyof

To Reproduce

In the mindset of a new user, type the example flipping ItemCar and ItemPlane and try to figure out why you don’t get the expected result for item2.

Expected behavior

At the very least there should be a note explaining that the order of Union variants are important in this context, and say how and why.

[tiangolo]

Awesome @kbanc ! 🚀 Thanks a lot for checking it and replicating the problem here, it helps a lot! 🎉 🙇

So, yes, that’s part of how Pydantic works, it’s documented here: https://pydantic-docs.helpmanual.io/usage/types/#unions

We could have a section in the docs in the first part that mentions Union (I’m not sure where is the first mention of Unions 🤔 ) with a warning about that.

Would you like to take a stab at that? 😬 😅 🤓 📝

[github-actions[bot]]

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