Resolvers

Every Field has a resolver function that is called to get the output value for the field. This can be simple, like accessing an attribute by name (the default), or can be something more complex like a computation, database call, or even a request to another API.

Signature

Each resolver callable must take the same three arguments. The resolver must return data that matches (or can be coerced by) the field’s type.

  • parent - The object that was resolved to get to this field. For example the parent of a username field might be an instance of a User class.

  • info - The GraphQLResolveInfo that describes the current operation. This has an attribute context that can be used to store arbitrary data, which can be useful for passing around a shared database connection or cache.

  • **kwargs - The values passed for any arguments on the field. By the time the resolver is called, GraphQL-Core will have checked the type and non-null of each argument, and Magql will have run Input Validation. You can write out individual argument names instead of **, but static type checking tools may not handle that correctly.

The resolver can raise ValidationError to show an error in the result instead of data. You’ll typically want to use the full validation system described below and at Input Validation instead of writing it all in the resolver.

Specifying a Resolver

There are a few different ways to define a Field with its resolver. A default resolver is used if none is given, described in the next section.

When creating the field, a resolver callable can be passed as the resolve parameter.

import magql

def resolve_greet(parent, info, **kwargs):
    return "Hello, World!"

field = magql.Field("String", resolve=resolve_greet)

If the field was already defined, you can assign a new resolver by decorating the function with Field.resolver() (or passing the function to it).

import magql

field = magql.Field("String")

@field.resolver
def resolve_greet(parent, info, **kwargs):
    return "Hello, World!"

# or
field.resolver(resolve_greet)

If you have an Object defined, you can decorate a resolver with Object.field() to add a field and its resolver simultaneously.

import magql

User = magql.Object("User")

@User.field("greet", "String")
def resolve_user_greet(parent, info, **kwargs):
    return f"Hello, {parent.username}!"

Default Resolver

If you don’t specify the resolver for a field, the default resolver resolve_attr() accesses the attribute with the same name as the field from the parent object, like getattr(parent, field_name). If instead of objects your data is dicts, you can specify resolve_item() instead, which does parent[field_name].

This default is much simpler than the one provided by GraphQL-Core. Core’s default will try the attribute, fall back to the item, then check if the value is callable and call it if so. While trying multiple ways is convenient if you have mixed data, Magql assumes you’re more likely to be working with dataclasses or database models, and that those classes were written without GraphQL in mind so their methods don’t act like resolvers.

Input Validation

Magql provides a robust input validation system beyond what GraphQL specifies. When resolving a field, Magql will run any registered validators for the field and its input structure before calling the field’s resolver. If validation fails, the resolver will not be called, and an error will be shown in the result.

See Input Validation.

Running Code Before Resolving

You may want to run some code before the validators or resolvers. Some use cases include keeping an audit log of queries, and checking authorization before processing or querying any data. In this case, you can add a “pre-resolve” function to the field. This takes the same arguments as a resolver callable (described above), but does not return a value. It may raise ValidationError to stop resolving early.

import logging
import magql

logger = logging.getLogger(__name__)
field = magql.Field("String")

@field.pre_resolver
def audit_access(parent, info, **kwargs):
    logger.info("{'.'.join(info.path.as_list())} was accessed.")