Controllers can't return falsy values for GraphQL queries and mutations

[gfpacheco]

Bug report

As I'm writing a custom controller action I cannot return falsy values because resolvers-builder.js returns undefined instead.

Describe the bug

https://github.com/strapi/strapi/blob/master/packages/strapi-plugin-graphql/services/resolvers-builder.js#L51 and https://github.com/strapi/strapi/blob/master/packages/strapi-plugin-graphql/services/resolvers-builder.js#L117 ignore valid falsy values (false, '', 0) assigned to ctx.body and return values instead.

Steps to reproduce the behavior

1. Create a custom action in any controller and set the response to 0
2. Create a route so you can call it
3. Write the GraphQL schema and resolver
4. Call REST endpoint and get expected result
5. Call GraphQL query and see error

Expected behavior

GraphQL query should return the same result as REST entpoint.

Code snippets

my-controller.js:

module.exports = {
  action: async (ctx, next) => {
    ctx.body = 0;
  },
};

routes.json

{
  "routes": [
    {
      "method": "GET",
      "path": "/my-controller/action",
      "handler": "my-controller.action",
      "config": {
        "policies": []
      }
    }
  ]
}

schema.graphql.js

module.exports = {
  query: `
    action: Int!
  `,
  resolver: {
    Query: {
      action: {
        description: 'Return zero',
        resolver: 'application::my-controller.my-controller.action',
      },
    },
  },
};

System

• Node.js version: v14.15.0
• NPM version: 7.6.0
• Yarn version: v1.22.10
• Strapi version: 3.5.4
• Database: sqlite
• Operating system: MacOS

Additional context

This can be easily solved by checking ctx.body === undefined and I can make the PR if you feel like this is ok.

[kasonde]

Hi @gfpacheco the Strapi GraphQL plugin uses apollo-server-koa under the hood which requires that resolver responses are "returned" from a resolver as opposed to fetched from the request context. You can see this in action in our Customize the GraphQL Schema example where restaurant records are returned in this manner for the custom GraphQL query:

restaurantsByCategories: {
  description: 'Return the restaurants open by the category',
  resolverOf: 'application::restaurant.restaurant.findByCategories', // Will apply the same policy on the custom resolver as the controller's action `findByCategories`.
  resolver: async (obj, options, ctx) => {
    // ctx is the context of the Koa request.
    await strapi.controllers.restaurants.findByCategories(ctx);

    return ctx.body.restaurants || `There is no restaurant.`;
  },
},

The response that is returned is taken from ctx.body.

[gfpacheco]

@kasonde I'm not sure if you read my issue properly or you don't know how Strapi works.

When you're creating a GraphQL query, instead of writing the resolver function in the schema file, you can just tell Strapi which controller action to call, in my case application::my-controller.my-controller.action.

And when you do that, Strapi creates a wrapper function that serves as your resolver function, this function reads the value returned by the controller from ctx.body and returns it to apollo-server-koa.

The problem is that this wrapper function ignores falsy values and returns undefined here and here.

[kasonde]

Hi @gfpacheco you're right. I've gone ahead and reopened the issue as well. Thanks again for your clarification.