How to pass and validate dynamic number of filter groups via query parameters?

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Example Code

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Description

I’ve searched for a solution for this seemingly common problem in APIs and have not found an answer. Please point me to the right direction. I’m trying to add dynamic filtering to my api endpoint by passing groups of filters via query parameters:

Say we have a table Person:

id
name: str
age: int
height: int
born: date

I want to create an API endpoint to get list of Persons using dynamic amount of filters

Let’s say we define each filter by having three properties:

field
operator
value

depending on type of field we are filtering, we’d have different operators:

str: begins, contains, ends
int, date: <, <=, =, >=, >

For example:

filters = {
    {field: "name", operator: "contains", value: "test"},
    {field: "age", operator: ">", value: 18},
    {field: "age", operator: "<=", value: 31},
    {field: "height", operator: ">", value: 150},
    {field: "height", operator: ">", value: 170},
}

OR

filters = {
    {field: "name", operator: "contains", value: "test"},
    {field: "age", operator: ">", value: 18},
}

As you can see number of filters is dynamic and can include same field several times.

I need to come up with a way to convert these filters into query parameters and pass it to FastAPI

I would like to have pydantic check that “field” value of each filter group is one of those allowed

Also need to make sure that based on type of field, only appropriate operators will be used

Then I could loop through each filter and apply them to query.

How can I achieve this with FastAPI?

How would filters be represented in URI? (I would like to avoid sending them via GET request body)

Is there an alternative way to achieve this?

Operating System

Linux, Windows, macOS

Operating System Details

No response

FastAPI Version

0.70.1

Python Version

3.9.9

Additional Context

No response

[dstlny]

@dstlny your suggestion does not work, the api expects the query filters list in the Body of the request and not Query params

Changing

filters: typing.List[QueryFilter] 

to

filters: typing.List[QueryFilter] = Depends(typing.List[QueryFilter])

should do the trick iirc

I’ve been thinking, and you’re probably better off stringifying the list on the frontend, and then parsing it after you recieve the stringified object, rather than letting FastAPI do it. Something like"

from fastapi import Depends
from pydantic import BaseModel, root_validator, parse_obj_as
import typing
import datetime
import json
from dateutil import parser

class QueryFilter(BaseModel):
	field: str
	operator: typing.Literal["includes", "contains", "<", "<=", ">=", ">"]
	value: typing.Union[str, int, datetime.date]

	@root_validator
	def validate_value(cls, values):
		
		operator, value = values.get("operator"), values.get("value")

		## is int
		if value.isdigit():
			if operator in {"<=", "<", ">", ">="}: ## etc
				pass
		else:
                       ## can't be any of these
			if operator in {"<=", "<", ">", ">="}: ## etc
				raise ValueError("Some error")

		return values

@router.get("/thing/")
async def thing(
	request: Request,
	filters: str = Query(...) ## pass as stringifed list of objects
):
	newFilter = json.loads(filters) ## load the stringified list

	all_filters = parse_obj_as(typing.List[QueryFilter], newFilter) ## let pydantic parse the JSON we just loaded... it''ll throw the same errors tha FastAPPI is expecting if it meets any errors

	print(all_filters) ## prints a list of QueryFilter

	return {}

Just tested this locally, stringifying from the frontend as such, and it works as it should. FastAPI get params really aren’t made for this kinda thing… 😃

[tiangolo]

Thanks for the help here everyone! 👏 🙇

Thanks for reporting back and closing the issue @jimbotrue 👍