Need help regarding creating a service for a sub collection in MongoDB in Feathers v5

[gypsicoder]

Suppose I have a collection named "post" in Mongodb for which I have created a service named "post". And each "post" item has a property named "comments" which is an array containing "comment" objects. Now, I want to create a separate service for "comments". Is it possible in Feathers v5?
For example the API URL might be something like this post/[postId]/comments or comments?postId=[postId]
I couldn't find any example for this type of problem. Someone please help!!!

[daffl]

Both is possible with a custom service. You can register it on the post/:postId/comments path and query this.app.service('post') for params.route.postId or on /comments and use params.query.postId. Then you'd implement the service methods to manipulate the comments for each post accordingly.

[gypsicoder]

@daffl
Thank you for your guidelines. I have made the service "post" with mongodb collection named "post" and set that model. Now, I am confused how to set the model for "comments"? And if I write a custom service how to do the pagination, sort and select stuffs?
Can you please show me some example with writing custom service for this type of scenario? I didn't find any in the FeathersJS website.

[gypsicoder]

It's a requirement to do in this way. I had been successful to create the service using separate collection which is very straight forward. But, I need the solution for the given scenario.

[daffl]

Have you tried generating a custom service with what I suggested? It scaffolds a lot of things for you and then you need to implement the logic you need for each method. For example to return all comments for a post in find it would be something like this:

export class CommentsService<ServiceParams extends CommentsParams = CommentsParams>
  implements ServiceInterface<Comments, CommentsData, ServiceParams, CommentsPatch>
{
  constructor(public options: CommentsServiceOptions) {}

  async find(params: ServiceParams = {}): Promise<Comments[]> {
    // via `?postId=<postId>` query parameter
    const { postId } = params.query || {}
    // or on the `post/:postId/comment route
    const { postId } = params.route || {}
    
    const { comments } = await this.options.app.service('posts').get(postId)

    return comments
  }

  async get(id: Id, _params?: ServiceParams): Promise<Comments> {
    return {
      id: 0,
      text: `A new message with ID: ${id}!`
    }
  }

  async create(data: CommentsData, params?: ServiceParams): Promise<Comments>
  async create(data: CommentsData[], params?: ServiceParams): Promise<Comments[]>
  async create(data: CommentsData | CommentsData[], params?: ServiceParams): Promise<Comments | Comments[]> {
    if (Array.isArray(data)) {
      return Promise.all(data.map((current) => this.create(current, params)))
    }

    return {
      id: 0,
      ...data
    }
  }

  // This method has to be added to the 'methods' option to make it available to clients
  async update(id: NullableId, data: CommentsData, _params?: ServiceParams): Promise<Comments> {
    return {
      id: 0,
      ...data
    }
  }

  async patch(id: NullableId, data: CommentsPatch, _params?: ServiceParams): Promise<Comments> {
    return {
      id: 0,
      text: `Fallback for ${id}`,
      ...data
    }
  }

  async remove(id: NullableId, _params?: ServiceParams): Promise<Comments> {
    return {
      id: 0,
      text: 'removed'
    }
  }
}

[gypsicoder]

Thank you for this. I have came to this point. But, I don't know how to do the sort, pagination for the comments here. Would you can show that also?

[daffl]

MongoDB does not have an option to do sorting and pagination with embedded arrays. You will have to do that manually yourself (e.g. by slicing the array based on the $sort, $limit and $skip parameters).

[gypsicoder]

@daffl
I have found a solution to do sort, and pagination using slice on mongo using Aggregate Framework