Field extensions

Field extensions are a great way to implement reusable logic such as permissions or pagination outside your resolvers. They wrap the underlying resolver and are able to modify the field and all arguments passed to the resolver.

📝 Note

The following examples only cover sync execution. To use extensions in async contexts, please have a look at Async Extensions and Resolvers

import strawberry
from strawberry.extensions import FieldExtension




class UpperCaseExtension(FieldExtension):
    def resolve(self, next_: Callable[..., Any], source: Any, info: Info, **kwargs):
        result = next_(source, info, **kwargs)
        return str(result).upper()




@strawberry.type
class Query:
    @strawberry.field(extensions=[UpperCaseExtension()])
    def string(self) -> str:
        return "This is a test!!"

In this example, the UpperCaseExtension wraps the resolver of the string field (next) and modifies the resulting string to be uppercase. The extension will be called instead of the resolver and receives the resolver function as the next argument. Therefore, it is important to not modify any arguments that are passed to next in an incompatible way.

Query
query {
    string
}
Response
{
  "string": "THIS IS A TEST!!"
}

Modifying the field

⚠️ Warning

The StrawberryField API is not stable and might change in the future without warning.

In some cases, the extended field needs to be compatible with the added extension. FieldExtension provides an apply(field: StrawberryField) method that can be overriden to modify the field. It is called during Schema Conversion. In the following example, we use apply to add a directive to the field:

import time
import strawberry
from strawberry.extensions import FieldExtension
from strawberry.schema_directive import Location
from strawberry.field import StrawberryField




@strawberry.schema_directive(locations=[Location.FIELD_DEFINITION])
class Cached:
    time: int = 100




class CachingExtension(FieldExtension):
    def __init__(self, caching_time=100):
        self.caching_time = caching_time
        self.last_cached = 0.0
        self.cached_result = None


    def apply(self, field: StrawberryField) -> None:
        field.directives.append(Cached(time=self.caching_time))


    def resolve(
        self, next_: Callable[..., Any], source: Any, info: Info, **kwargs
    ) -> Any:
        current_time = time.time()
        if self.last_cached + self.caching_time > current_time:
            return self.cached_result
        self.cached_result = next_(source, info, **kwargs)
        return self.cached_result
Python
@strawberry.type
class Client:


    @strawberry.field(extensions=[CachingExtensions(caching_time=200)])
    def analyzed_hours(self, info) -> int:
        return do_expensive_computation()
Schema
type Client {
    analyzedHours: Int! @Cached(time=200)
}

Combining multiple field extensions

When chaining multiple field extensions, the last extension in the list is called first. Then, it calls the next extension until it reaches the resolver. The return value of each extension is passed as an argument to the next extension. This allows for creating a chain of field extensions that each perform a specific transformation on the data being passed through them.

@strawberry.field(extensions=[LowerCaseExtension(), UpperCaseExtension()])
def my_field():
    return "My Result"
💡 Tip

Order matters: the last extension in the list will be executed first, while the first extension in the list extension will be applied to the field first. This enables cases like adding relay pagination in front of an extension that modifies the field's type.

Async Extensions and Resolvers

Field Extensions support async execution using the resolve_async method. A field extension can either support sync, async, or both. The appropriate resolve function will be automatically chosen based on the type of resolver and other extensions.

Since sync-only extensions cannot await the result of an async resolver, they are not compatible with async resolvers or extensions.

The other way around is possible: you can add an async-only extension to a sync resolver, or wrap sync-only extensions with it. This is enabled by an automatic use of the SyncToAsyncExtension. Note that after adding an async-only extension, you cannot wrap it with a sync-only extension anymore.

💡 Tip

To optimize the performance of your resolvers, it's recommended that you implement both the resolve and resolve_async methods when using an extension on both sync and async fields. While the SyncToAsyncExtension is convenient, it may add unnecessary overhead to your sync resolvers, leading to slightly decreased performance.

import strawberry
from strawberry.extensions import FieldExtension




class UpperCaseExtension(FieldExtension):
    def resolve(self, next: Callable[..., Any], source: Any, info: Info, **kwargs):
        result = next(source, info, **kwargs)
        return str(result).upper()


    async def resolve_async(
        self, next: Callable[..., Awaitable[Any]], source: Any, info: Info, **kwargs
    ):
        result = await next(source, info, **kwargs)
        return str(result).upper()




@strawberry.type
class Query:
    @strawberry.field(extensions=[UpperCaseExtension()])
    async def string(self) -> str:
        return "This is a test!!"

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