Middleware¶

You can use middleware to affect the evaluation of fields in your schema.

A middleware is any object or function that responds to resolve(next_middleware, *args).

Inside that method, it should either:

Send resolve to the next middleware to continue the evaluation; or
Return a value to end the evaluation early.

Resolve arguments¶

Middlewares resolve is invoked with several arguments:

next represents the execution chain. Call next to continue evaluation.
root is the root value object passed throughout the query.
info is the resolver info.
args is the dict of arguments passed to the field.

Example¶

This middleware only continues evaluation if the field_name is not 'user'

class AuthorizationMiddleware(object):
    def resolve(self, next, root, info, **args):
        if info.field_name == 'user':
            return None
        return next(root, info, **args)

And then execute it with:

result = schema.execute('THE QUERY', middleware=[AuthorizationMiddleware()])

If the middleware argument includes multiple middlewares, these middlewares will be executed bottom-up, i.e. from last to first.

Functional example¶

Middleware can also be defined as a function. Here we define a middleware that logs the time it takes to resolve each field:

from time import time as timer

def timing_middleware(next, root, info, **args):
    start = timer()
    return_value = next(root, info, **args)
    duration = round((timer() - start) * 1000, 2)
    parent_type_name = root._meta.name if root and hasattr(root, '_meta') else ''
    logger.debug(f"{parent_type_name}.{info.field_name}: {duration} ms")
    return return_value

And then execute it with:

result = schema.execute('THE QUERY', middleware=[timing_middleware])

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