### Install moleculer-apollo-server

Source: https://github.com/moleculerjs/moleculer-apollo-server/blob/master/README.md

Install the necessary packages for moleculer-apollo-server, moleculer-web, and graphql.

```bash
npm i moleculer-apollo-server moleculer-web graphql
```

--------------------------------

### Generated GraphQL Schema

Source: https://github.com/moleculerjs/moleculer-apollo-server/blob/master/README.md

This is an example of the GraphQL schema automatically generated based on the service actions defined with the 'graphql' property.

```graphql
type Mutation {
  welcome(name: String!): String
}

type Query {
  hello: String
}

```

--------------------------------

### Define GraphQL Queries and Mutations in Service Actions

Source: https://github.com/moleculerjs/moleculer-apollo-server/blob/master/README.md

Define GraphQL queries and mutations directly within Moleculer service action definitions using the 'graphql' property. This example shows a 'hello' query and a 'welcome' mutation.

```javascript
module.exports = {
    name: "greeter", 

    actions: {
        hello: {
            graphql: {
                query: "hello: String"
            },
            handler(ctx) {
                return "Hello Moleculer!"
            }
        },
        welcome: {
            params: {
                name: "string"
            },
            graphql: {
                mutation: "welcome(name: String!): String"
            },
            handler(ctx) {
                return `Hello ${ctx.params.name}`;
            }
        }
    }
};

```

--------------------------------

### Setup API Gateway with ApolloService Mixin

Source: https://github.com/moleculerjs/moleculer-apollo-server/blob/master/README.md

Configure the Moleculer API Gateway to handle GraphQL requests using the ApolloService mixin. This sets up the default '/graphql' endpoint.

```javascript
"use strict";

const ApiGateway 	= require("moleculer-web");
const { ApolloService } = require("moleculer-apollo-server");

module.exports = {
    name: "api",

    mixins: [
        // Gateway
        ApiGateway,

        // GraphQL Apollo Server
        ApolloService({

            // Global GraphQL typeDefs
            typeDefs: ``, 

            // Global resolvers
            resolvers: {},

            // API Gateway route options
            routeOptions: {
                path: "/graphql",
                cors: true,
                mappingPolicy: "restrict"
            },

            // https://www.apollographql.com/docs/apollo-server/api/apollo-server#options
            serverOptions: {}
        })
    ]
};

```

--------------------------------

### Define Post Type and Resolvers in posts.service.js

Source: https://github.com/moleculerjs/moleculer-apollo-server/blob/master/README.md

Defines the GraphQL 'Post' type and configures resolvers to fetch related 'author' and 'voters' by calling the 'users.resolve' action. This setup is useful for linking entities across services.

```javascript
module.exports = {
    name: "posts",
    settings: {
        graphql: {
            type: `
                """
                This type describes a post entity.
                """		
type Post {
                    id: Int!
                    title: String!
                    author: User!
                    votes: Int!
                    voters: [User]
                    createdAt: Timestamp
                }
            `,
            resolvers: {
                Post: {
                    author: {
                        // Call the `users.resolve` action with `id` params
                        action: "users.resolve",
                        rootParams: {
                            "author": "id"
                        }
                    },
                    voters: {
                        // Call the `users.resolve` action with `id` params
                        action: "users.resolve",
                        rootParams: {
                            "voters": "id"
                        }
                    }
                }
            }
        }
    },
    actions: {
        find: {
            //cache: true,
            params: {
                limit: { type: "number", optional: true }
            },
            graphql: {
                query: `posts(limit: Int): [Post]`
            },
            handler(ctx) {
                let result = _.cloneDeep(posts);
                if (ctx.params.limit)
                    result = posts.slice(0, ctx.params.limit);
                else
                    result = posts;

                return _.cloneDeep(result);
            }
        },

        findByUser: {
            params: {
                userID: "number"
            },
            handler(ctx) {
                return _.cloneDeep(posts.filter(post => post.author == ctx.params.userID));
            }
        },
    }
};
```

--------------------------------

### Define User Type and Resolvers in users.service.js

Source: https://github.com/moleculerjs/moleculer-apollo-server/blob/master/README.md

Defines the GraphQL 'User' type and configures resolvers for 'posts' and 'postCount'. It demonstrates calling 'posts.findByUser' to retrieve a user's posts and 'posts.count' to get the number of posts by a specific author.

```javascript
module.exports = {
    name: "users",
    settings: {
        graphql: {
            type: `
                """
                This type describes a user entity.
                """		
type User {
                    id: Int!
                    name: String!
                    birthday: Date
                    posts(limit: Int): [Post]
                    postCount: Int
                }
            `,
            resolvers: {
                User: {
                    posts: {
                        // Call the `posts.findByUser` action with `userID` param.
                        action: "posts.findByUser",
                        rootParams: {
                            "id": "userID"
                        }
                    },
                    postCount: {
                        // Call the "posts.count" action
                        action: "posts.count",
                        // Get `id` value from `root` and put it into `ctx.params.query.author`
                        rootParams: {
                            "id": "query.author"
                        }
                    }			
                }
            }			
        }
    },
    actions: {
        find: {
            //cache: true,
            params: {
                limit: { type: "number", optional: true }
            },
            graphql: {
                query: "users(limit: Int): [User]"
            },
            handler(ctx) {
                let result = _.cloneDeep(users);
                if (ctx.params.limit)
                    result = users.slice(0, ctx.params.limit);
                else
                    result = users;

                return _.cloneDeep(result);
            }
        },

        resolve: {
            params: {
                id: [
                    { type: "number" },
                    { type: "array", items: "number" }
                ]
            },
            handler(ctx) {
                if (Array.isArray(ctx.params.id)) {
                    return _.cloneDeep(ctx.params.id.map(id => this.findByID(id)));
                } else {
                    return _.cloneDeep(this.findByID(ctx.params.id));
                }
            }
        }
    }
};
```

--------------------------------

### Configure DataLoader Options in Action

Source: https://github.com/moleculerjs/moleculer-apollo-server/blob/master/README.md

DataLoader options, such as `maxBatchSize`, can be specified in the called action definition's `graphql` property. The action handler must be able to process both single IDs and arrays of IDs.

```javascript
resolve: {
    params: {
        id: [{ type: "number" }, { type: "array", items: "number" }],
        graphql: { dataLoaderOptions: { maxBatchSize: 100 } },
    },
    handler(ctx) {
        this.logger.debug("resolve action called.", { params: ctx.params });
        if (Array.isArray(ctx.params.id)) {
            return _.cloneDeep(ctx.params.id.map(id => this.findByID(id)));
        } else {
            return _.cloneDeep(this.findByID(ctx.params.id));
        }
    },
},

```

--------------------------------

### Enable DataLoader for Resolver

Source: https://github.com/moleculerjs/moleculer-apollo-server/blob/master/README.md

To enable DataLoader for a resolver, add `dataLoader: true` to the resolver's property object within the `resolvers` configuration of the service's `graphql` property. This requires the called action to accept params with an array property and return an array of the same size.

```javascript
module.exports = {
    settings: {
        graphql: {
            resolvers: {
                Post: {
                    author: {
                        action: "users.resolve",
                        dataLoader: true,
                        rootParams: {
                            author: "id",
                        },
                    },
                    voters: {
                        action: "users.resolve",
                        dataLoader: true,
                        rootParams: {
                            voters: "id",
                        },
                    },
                    // ...
                }
            }
        }
    }
};        
```

--------------------------------

### users.resolve Action

Source: https://github.com/moleculerjs/moleculer-apollo-server/blob/master/README.md

Resolves one or more user entities by their IDs. This action is used internally by resolvers.

```APIDOC
## users.resolve

### Description
Resolves user(s) by their unique identifier(s).

### Parameters
#### Path Parameters
- **id** (number | number[]) - Required - The ID or an array of IDs of the user(s) to resolve.

### Response
#### Success Response (200)
- **User** (User) or **[User]** ([User]) - The resolved user object or an array of user objects.

### Request Example
```json
{
  "id": 101
}
```

### Request Example (Array)
```json
{
  "id": [101, 102]
}
```

### Response Example
```json
{
  "id": 101,
  "name": "John Doe"
}
```

### Response Example (Array)
```json
[
  {
    "id": 101,
    "name": "John Doe"
  },
  {
    "id": 102,
    "name": "Jane Smith"
  }
]
```
```

--------------------------------

### Posts Service GraphQL API

Source: https://github.com/moleculerjs/moleculer-apollo-server/blob/master/README.md

Exposes GraphQL queries for fetching post data, including a `posts` query that can be filtered by a limit.

```APIDOC
## posts

### Description
Fetches a list of posts, optionally limited to a specified number.

### Method
GET

### Endpoint
/graphql

### Parameters
#### Query Parameters
- **limit** (Int) - Optional - The maximum number of posts to return.

### Response
#### Success Response (200)
- **posts** ([Post]) - A list of Post objects.

### Request Example
```graphql
query {
  posts(limit: 10) {
    id
    title
    author {
      id
      name
    }
    votes
    createdAt
  }
}
```

### Response Example
```json
{
  "data": {
    "posts": [
      {
        "id": 1,
        "title": "First Post",
        "author": {
          "id": 101,
          "name": "John Doe"
        },
        "votes": 5,
        "createdAt": "2023-10-27T10:00:00Z"
      }
    ]
  }
}
```
```

--------------------------------

### Users Service GraphQL API

Source: https://github.com/moleculerjs/moleculer-apollo-server/blob/master/README.md

Exposes GraphQL queries for fetching user data, including a `users` query that can be filtered by a limit.

```APIDOC
## users

### Description
Fetches a list of users, optionally limited to a specified number.

### Method
GET

### Endpoint
/graphql

### Parameters
#### Query Parameters
- **limit** (Int) - Optional - The maximum number of users to return.

### Response
#### Success Response (200)
- **users** ([User]) - A list of User objects.

### Request Example
```graphql
query {
  users(limit: 5) {
    id
    name
    birthday
  }
}
```

### Response Example
```json
{
  "data": {
    "users": [
      {
        "id": 101,
        "name": "John Doe",
        "birthday": "1990-05-15"
      }
    ]
  }
}
```
```

--------------------------------

### User.postCount Resolver

Source: https://github.com/moleculerjs/moleculer-apollo-server/blob/master/README.md

Resolves the `postCount` field for a User type by calling the `posts.count` action.

```APIDOC
## User.postCount

### Description
Retrieves the total number of posts authored by a specific user.

### Method
GET

### Endpoint
/graphql

### Response
#### Success Response (200)
- **postCount** (Int) - The total count of posts authored by the user.

### Request Example
```graphql
query {
  users {
    id
    postCount
  }
}
```

### Response Example
```json
{
  "data": {
    "users": [
      {
        "id": 101,
        "postCount": 15
      }
    ]
  }
}
```
```

--------------------------------

### User.posts Resolver

Source: https://github.com/moleculerjs/moleculer-apollo-server/blob/master/README.md

Resolves the `posts` field for a User type by calling the `posts.findByUser` action.

```APIDOC
## User.posts

### Description
Retrieves all posts authored by a specific user.

### Method
GET

### Endpoint
/graphql

### Parameters
#### Query Parameters
- **userID** (Int) - Required - The ID of the user whose posts are to be fetched.

### Response
#### Success Response (200)
- **posts** ([Post]) - A list of Post objects authored by the user.

### Request Example
```graphql
query {
  users {
    id
    posts(limit: 3) {
      id
      title
    }
  }
}
```

### Response Example
```json
{
  "data": {
    "users": [
      {
        "id": 101,
        "posts": [
          {
            "id": 1,
            "title": "First Post"
          },
          {
            "id": 2,
            "title": "Second Post"
          }
        ]
      }
    ]
  }
}
```
```

--------------------------------

### Post.author Resolver

Source: https://github.com/moleculerjs/moleculer-apollo-server/blob/master/README.md

Resolves the `author` field for a Post type by calling the `users.resolve` action.

```APIDOC
## Post.author

### Description
Retrieves the author of a specific post.

### Method
GET

### Endpoint
/graphql

### Response
#### Success Response (200)
- **author** (User) - The User object representing the author of the post.

### Request Example
```graphql
query {
  posts {
    id
    author {
      id
      name
    }
  }
}
```

### Response Example
```json
{
  "data": {
    "posts": [
      {
        "id": 1,
        "author": {
          "id": 101,
          "name": "John Doe"
        }
      }
    ]
  }
}
```
```

--------------------------------

### Post.voters Resolver

Source: https://github.com/moleculerjs/moleculer-apollo-server/blob/master/README.md

Resolves the `voters` field for a Post type by calling the `users.resolve` action.

```APIDOC
## Post.voters

### Description
Retrieves the list of users who have voted on a specific post.

### Method
GET

### Endpoint
/graphql

### Response
#### Success Response (200)
- **voters** ([User]) - A list of User objects representing the voters.

### Request Example
```graphql
query {
  posts {
    id
    voters {
      id
      name
    }
  }
}
```

### Response Example
```json
{
  "data": {
    "posts": [
      {
        "id": 1,
        "voters": [
          {
            "id": 101,
            "name": "John Doe"
          },
          {
            "id": 102,
            "name": "Jane Smith"
          }
        ]
      }
    ]
  }
}
```
```

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