feat(utils): makeWrapResolversPlugin, makeChangeNullabilityPlugin

[benjie]

Fixes #342

e.g. to lowercase all user emails:

makeWrapResolversPlugin({
  User: {
    async email(resolve, source, args, context, resolveInfo) {
      const result = await resolve();
      return result.toLowerCase();
    }
  }
});

e.g. to only allow the user to see their own email:

// 1: make User.email nullable:
makeChangeNullabilityPlugin({
  User: {
    email: true,
  },
}),
// 2: return null unless the user id matches the current logged in user_id
makeWrapResolversPlugin({
  User: {
    email: {
      requires: {
        siblingColumns: [{ column: "id", alias: "$user_id" }],
      },
      resolve(resolver, user, args, context, _resolveInfo) {
        if (context.jwtClaims.user_id !== user.$user_id) return null;
        return resolver();
      },
    },
  },
}),

e.g. perform validation when registering a user and take an action afterwards:

makeWrapResolversPlugin({
  Mutation: {
    createUser: {
      requires: {
        childColumns: [{ column: "id", alias: "$new_id" }],
      },
      async resolve(resolver, mutation, args, _context, _resolveInfo) {
		/* Optional: VALIDATION */
        const {
          input: {
            user: { name },
          },
        } = args;
        if (name.length < 3) throw new Error("Name is too short");
        /* End: VALIDATION */

        const result = await resolver();

        /* Optional: ACTION */
        const { data: { $new_id: newUserId } } = result;
        await doAThingForUserId(newUserId); //e.g. send welcome email
        /* End: ACTION */

        return result;
      },
    },
  },
}),

You can handle lots of types and fields in the same plugin.

Noop example:

module.exports = makeWrapResolversPlugin({
  TypeNameHere: {
    fieldNameHere: resolve => resolve()
  }
});

Generic example:

module.exports = makeWrapResolversPlugin({
  TypeNameHere: {
    async fieldNameHere(resolve, source, args, context, resolveInfo) {
      // Do something before
      const result = await resolve(
        source, // optional, specify when overriding
        args, // optional, specify when overriding
        context, // optional, specify when overriding
        resolveInfo // optional, specify when overriding
      );
      // Do something after
      return result;
    },
    anotherFieldNameHere(...) { ... }
  },
  AnotherTypeNameHere: ...
});