silverstripe-framework/docs/en/02_Developer_Guides/19_GraphQL/03_working_with_generic_types/03_resolver_discovery.md

title	summary
The resolver discovery pattern	How you can opt out of mapping fields to resolvers by adhering to naming conventions

Working with generic types

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The resolver discovery pattern

When you define a query, mutation, or any other field on a type, you can opt out of providing an explicit resolver and allow the system to discover one for you based on naming convention.

Let's start by registering a resolver class where we can define a bunch of these methods.

You can register as many classes as makes sense - and each resolver class can have multiple resolver methods.

app/_graphql/config.yml

resolvers:
  - MyProject\Resolvers\MyResolvers

What we're registering here is a generic class that should contain one or more static functions that resolve one or many fields. How those functions will be discovered relies on the resolver strategy.

Resolver strategy

Each schema config accepts a resolverStrategy property. This should map to a callable that will return a method name given a class name, type name, and Field instance.

class Strategy
{
    public static function getResolverMethod(string $className, ?string $typeName = null, ?Field $field = null): ?string
    {
        // strategy logic here
    }
}

The default resolver strategy

By default, all schemas use DefaultResolverStrategy::getResolverMethod() to discover resolver functions. The logic works like this:

• Does resolve<TypeName><FieldName> exist?
  • Yes? Return that method name
  • No? Continue
• Does resolve<TypeName> exist?
  • Yes? Return that method name
  • No? Continue
• Does resolve<FieldName> exist?
  • Yes? Return that method name
  • No? Continue
• Does resolve exist?
  • Yes? Return that method name
  • No? Return null. This resolver cannot be discovered

Let's look at our query again:

query {
  readCountries {
    name
  }
}

Imagine we have two classes registered under resolvers - ClassA and ClassB

app/_graphql/config.yml

resolvers:
  - ClassA
  - ClassB

The DefaultResolverStrategy will check for methods in this order:

• ClassA::resolveCountryName()
• ClassA::resolveCountry()
• ClassA::resolveName()
• ClassA::resolve()
• ClassB::resolveCountryName()
• ClassB::resolveCountry()
• ClassB::resolveName()
• ClassB::resolve()
• Return null.

You can implement whatever strategy you like in your schema. Just register it to resolverStrategy in the config.

app/_graphql/config.yml

resolverStrategy: [ 'MyApp\Resolvers\Strategy', 'getResolverMethod' ]

Let's add a resolver method to our resolver provider:

app/src/Resolvers/MyResolvers.php

namespace MyApp\Resolvers;

class MyResolvers
{
    public static function resolveReadCountries()
    {
        $results = [];
        $countries = Injector::inst()->get(Locales::class)->getCountries();
        foreach ($countries as $code => $name) {
            $results[] = [
                'code' => $code,
                'name' => $name
            ];
        }

        return $results;
    }
}

Now that we're using logic to discover our resolver, we can remove our resolver method declarations from the individual queries and instead just register the resolver class.

app/_graphql/config.yml

resolvers:
  - MyApp\Resolvers\MyResolvers

app/_graphql/schema.yml

  queries:
    readCountries: '[Country]'

Re-run the schema build, with a flush (because we created a new PHP class), and let's go!

vendor/bin/sake dev/graphql/build schema=default flush=1

Field resolvers

A less magical approach to resolver discovery is defining a fieldResolver property on your types. This is a generic handler for all fields on a given type and can be a nice middle ground between the rigor of hard coding everything at a query level, and the opacity of discovery logic.

app/_graphql/schema.yml

  types:
    Country:
      fields:
        name: String
        code: String
      fieldResolver: [ 'MyProject\MyResolver', 'resolveCountryFields' ]

In this case the registered resolver method will be used to resolve any number of fields. You'll need to do explicit checks for the field name in your resolver to make this work.

public static function resolveCountryFields($obj, $args, $context, ResolveInfo $info)
{
    $fieldName = $info->fieldName;
    if ($fieldName === 'image') {
        return $obj->getImage()->getURL();
    }
    // .. etc
}

Further reading

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