How would you use asyncpg with FastAPI to map returned values from a select query to pydantic models for output and validation?

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I want to use FastAPI without an ORM (using asyncpg) and map the returned values from a select query to a pydantic model. This way the returned values are validated with pydantic and the response that is returned is structured like the pydantic model/schema.

I’ve tried looking for documentation on this but it’s pretty hard to find/not clear. I’d appreciate any help!

asyncpg pydantic fastapi python-3.x
1 Answers

Every pydantic model inherits a couple of utility helpers to create objects. One is parse_obj which takes a dict and creates the model object from that.

parse_obj: this is very similar to the __init__ method of the model, except it takes a dict rather than keyword arguments. If the object passed is not a dict a ValidationError will be raised.

From the example on the linked section above:

class User(BaseModel):
    id: int
    name = 'John Doe'
    signup_ts: datetime = None


m = User.parse_obj({'id': 123, 'name': 'James'})
print(m)
#> id=123 signup_ts=None name='James'

You might be able to give parse_obj a Record directly since it implements dict-like accessors, so just try it and see if it works. If not you can use dict(<row record from asyncpg>) to convert the record to an actual dict.

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