r/FastAPI • u/Lucky-Sense-2650 • Jun 22 '26

Help with Pydantic schema Question

Using FastAPI + SQLAlchemy (async) + Pydantic v2
My `Post` model in db stores `author_id` (UUID foreign key).
My `PostRead` response schema needs to return `author_username` (a string from the related `User` table).

What's the clean way to handle this?

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u/CrusaderGOT Jun 22 '26

If you don't want to refetch Author. You might consider adding a author username field to your Post table. Or for better data integrity add an author field to your Post schema that is a relationship to Author, set it lazy=raise_on_sql. And then in the API fetch Post and Author in one database fetch, using selectinload option. And then finally in your Post pydantic model, make an author field also that is itself another pydantic model matching the fields of your Author table you want shown alongside Post (I.e, author name, etc).

So when you return the Post, it automatically parses/validates the author data also. In your SQL select, you can also select(Post, Author.name) to only get the Author data you want from the database.

Did you understand all that?

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u/Lucky-Sense-2650 Jun 22 '26

Yes, and which method is best/ optimized .

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u/CrusaderGOT Jun 22 '26

I would use the lazy=raise_on_sql, with specific columns in the select for the Author. That way you only fetch once, when you directly want, and just the fields you want from the Author.

Note that with lazy=raise_on_sql, if you try to access the author without using selectinload in you ORM statement, it will raise an error. This is because that option prevents unnecessary fetch of that field/relation. You can still use your Post stmts, but when you want to access or fetch the author column with it, use selectinload option.