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🛡 Nexus plugin to ease the creation of the authorization layer

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# nexus-shield

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## Help Wanted ⚠️

If you are a Typescript expert, I could use a hand on a lingering typing issue when a shield parameter is added to an objectType.
Please see the related [issue](https://github.com/Sytten/nexus-shield/issues/50) for details. Thanks!

## Overview

Nexus Shield is a [nexus](https://github.com/graphql-nexus/nexus) plugin that helps you create an authorization layer for your application. It is a replacement for the provided authorization plugin. It is heavily inspired by [Graphql Shield](https://github.com/maticzav/graphql-shield) and reuses most of its familiar ruling system. It takes full advantage of the type safety provided by nexus.

## Install

```bash
npm install --save nexus-shield

OR

yarn add nexus-shield
```

## Usage

### Nexus configuration

The plugin first needs to be installed in nexus. This will add the new `shield` parameter. The plugin will work without any provided configuration, but it is recommended to provide one that is relevant to your application. The available parameters are:

- `defaultError`: The error that is thrown if the access is denied. See the [errors section](#Error).
- `defaultRule`: Rule that is used if none is specified for a field.
- `hashFunction`: Function used to hash the input to provide [caching keys](#Caching).

For example, using an [Apollo server](https://www.apollographql.com/server/):

```typescript
import { nexusShield, allow } from 'nexus-shield';
import { ForbiddenError } from 'apollo-server';

const schema = makeSchema({
  // ... Rest of the configuration
  plugins: [
    nexusShield({
      defaultError: new ForbiddenError('Not allowed'),
      defaultRule: allow,
    }),
  ],
});
```

#### Subscriptions configuration

- When using subscriptions with a server that is not integrated directly into your "main" GraphQL server, you **must** make sure that you pass in a valid context.
- This context should contain all the information needed to evaluate the rules. Ideally, it is the same as the context for your "main" server otherwise the typing won't reflect the data available to the rules.

For example, using [GraphQL-WS](https://github.com/enisdenjo/graphql-ws):

```typescript
useServer(
  {
    schema,
    context: (ctx, msg, args) => {
      // That will return the same context that was passed when the
      // server received the subscription request
      return ctx;
    },
  },
  wsServer
);
```

### Styles

Two interface styles are provided for convenience: `Graphql-Shield` and `Nexus`.

#### Graphql-Shield

```typescript
rule()((root, args, ctx) => {
  return !!ctx.user;
});
```

#### Nexus

```typescript
ruleType({
  resolve: (root, args, ctx) => {
    return !!ctx.user;
  },
});
```

### Error

- A rule needs to return a `boolean`, a `Promise<boolean>` or throw an `Error`.
- Contrary to Graphql-shield, this plugin will **NOT** catch the errors you throw and will just pass them down to the next plugins and eventually to the server
- If `false` is returned, the configured `defaultError` will be thrown by the plugin.

```typescript
import { AuthenticationError } from 'apollo-server';

const isAuthenticated = ruleType({
  resolve: (root, args, ctx) => {
    const allowed = !!ctx.user;
    if (!allowed) throw new AuthenticationError('Bearer token required');
    return allowed;
  },
});
```

### Operators

Rules can be combined in a very flexible manner. The plugin provides the following operators:

- `and`: Returns `true` if **all** rules return `true`
- `or`: Returns `true` if **one** rule returns `true`
- `not`: Inverts the result of a rule
- `chain`: Same as `and`, but rules are executed in order
- `race`: Same as `or`, but rules are executed in order
- `deny`: Returns `false`
- `allow`: Returns `true`

Simple example:

```typescript
import { chain, not, ruleType } from 'nexus-shield';

const hasScope = (scope: string) => {
  return ruleType({
    resolve: (root, args, ctx) => {
      return ctx.user.permissions.includes(scope);
    },
  });
};

const backlist = ruleType({
  resolve: (root, args, ctx) => {
    return ctx.user.token === 'some-token';
  },
});

const viewerIsAuthorized = chain(
  isAuthenticated,
  not(backlist),
  hasScope('products:read')
);
```

### Shield Parameter

To use a rule, it must be assigned to the `shield` parameter of a field:

```typescript
export const Product = objectType({
  name: 'Product',
  definition(t) {
    t.id('id');
    t.string('prop', {
      shield: ruleType({
        resolve: (root, args, ctx) => {
          return !!ctx.user;
        },
      }),
    });
  },
});
```

### Type safety

This plugin will try its best to provide typing to the rules.

- It is **preferable** to define rules directly in the `definition` to have access to the full typing of `root` and `args`.
- The `ctx` is always typed if it was properly configured in nexus `makeSchema`.
- If creating generic or partial rules, use the appropriate helpers (see below).

```typescript
export type Context = {
  user?: { id: string };
};

export const Product = objectType({
  name: 'Product',
  definition(t) {
    t.id('id');
    t.string('ownerId');
    t.string('prop', {
      args: {
        filter: stringArg({ nullable: false }),
      },
      shield: ruleType({
        resolve: (root, args, ctx) => {
          // root => { id: string }, args => { filter: string }, ctx => Context
          return true;
        },
      }),
    });
  },
});
```

#### Generic rules

- Generic rules are rules that do not depend on the type of the `root` or `args`.
- The wrapper `generic` is provided for this purpose. It will wrap your rule in a generic function.

```typescript
const isAuthenticated = generic(
  ruleType({
    resolve: (root, args, ctx) => {
      // Only ctx is typed
      return !!ctx.user;
    },
  })
);

// Usage
t.string('prop', {
  shield: isAuthenticated(),
});
```

#### Partial rules

- Partial rules are rules that depend only on the type of the `root`.
- The wrapper `partial` is provided for this purpose. It will wrap your rule in a generic function.

```typescript
const viewerIsOwner = partial(
  ruleType({
    type: 'Product' // It is also possible to use the generic parameter of `partial`
    resolve: (root, args, ctx) => {
      // Both root and ctx are typed
      return root.ownerId === ctx.user.id;
    },
  })
);

// Usage
t.string('prop', {
  shield: viewerIsOwner(),
});
```

#### Combining rules

If you mix and match generic rules with partial rules, you will need to specify the type in the parent helper.

```typescript
const viewerIsAuthorized = partial<'Product'>(
  chain(isAuthenticated(), viewerIsOwner())
);
```

However, if you specify it directly in the `shield` field, there is no need for a helper thus no need for a parameter.

```typescript
t.string('prop', {
  shield: chain(isAuthenticated(), viewerIsOwner()),
});
```

### Caching

- The result of a rule can be cached to maximize performance. This is important when using generic or partial rules that require access to external data.
- The caching is **always** scoped to the request

The plugin offers 3 levels of caching:

- `NO_CACHE`: No caching is done (default)
- `CONTEXTUAL`: Use when the rule only depends on the `ctx`
- `STRICT`: Use when the rule depends on the `root` or `args`

Usage:

```typescript
rule({ cache: ShieldCache.STRICT })((root, args, ctx) => {
  return true;
});

ruleType({
  cache: ShieldCache.STRICT,
  resolve: (root, args, ctx) => {
    return !!ctx.user;
  },
});
```

### Known issues / limitations

- Currently, the typing of the `shield` parameter on `objectType` doesn't work. Tracked by issue: https://github.com/Sytten/nexus-shield/issues/50

- It is not possible to pass directly an `objectType` to the parameter `type` of a `ruleType`. Tracked by issue: https://github.com/graphql-nexus/schema/issues/451

- The helpers are necessary to provide strong typing and avoid the propagation of `any`. See [this StackOverflow issue](https://stackoverflow.com/questions/62363077/combining-typescript-generics-with-any-without-losing-type/62435780#62435780) for more on the subject.

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