使用 angularfire2 查询 Firebase 实时数据库上的数据关系 [英] Query data relationship on Firebase Realtime Database with angularfire2

问题描述

我需要查询评论并仅请求通过 userId 在评论中列出的用户.

I need to query comments and request only user that listed in the comment by userId.

我在 Firebase 实时数据库中的数据库结构:

My database structure in Firebase realtime db:

{
  "comments" : {
    "c_id1" : {
      "commentId" : "c_id1",
      "commentText" : "text",
      "userId" : "u_id1"
    },
    "c_id2" : {
      "commentId" : "c_id2",
      "commentText" : "text",
      "userId" : "u_id3"
    },
  },

  "users" : {
    "u_id1" : {
      "userId" : "u_id1",
      "userName" : "name1",
    },
    "u_id1" : {
      "userId" : "u_id2",
      "userName" : "name2",
    },
    "u_id1" : {
      "userId" : "u_id3",
      "userName" : "name3",
    }
  }
}

我最后需要的是Comment[]，其中Comment是:

What I need in the end is Comment[], where Comment is:

{
  "commentId" : "c_id",
  "commentText" :"text",
  "userId" : "u_id",
  "user" : {
    "userId":"u_id",
    "userName":"name"
  }
}

所以，Comment 的类是

so, the class for Comment is

export class Comment {
  commentId: string;
  commentText: string;
  userId: string;
  user?: User;
}

到目前为止，我设法获取了所有用户，然后将它们映射到客户端的评论.但是，如果 db 有 N 个用户，只有 2 个评论，其中 N>>2，那岂不是太多了?

So far I managed to get ALL users and then map them to comments on the client side. But wouldn't it be to much in case when db will have N number of users and only 2 comments, where N>>2?

  OnGetUsersForComments(){
    return this.angularFireDatabase.list("/comments").valueChanges()
      .subscribe((data) => {
        this.commentsUsers = data;
        this.OnGetCommentsForTask()
      });
  }

  OnGetCommentsForTask(){
    this.angularFireDatabase.list("/comments").valueChanges()
      .map((comments) => {
        return comments.map( (comment: TaskComment) => {
          this.commentsUsers.forEach((user: User) => {
            if (comment.userId === user.userId) {
              comment.commentUser = user;
            }
          });
          return comment;
        });
      })
      .subscribe((data)=> {
        this.comments = data;
      });
  }

有没有办法只从评论中获取用户?

Is there a way get only users from comments?

我也尝试将其添加到用户中，但未能使其正常工作:

I also tried to add this to the User, but did not manage it to work:

"userComments" : {
  "uc_id1" : {
    "commentId" : c_id2
  },
}

<小时>

更新 0

我已经编辑了问题，我希望现在更清楚了.

I have edited the question, I hope now is more clear.

我已经能够让它像这样工作:来自 - https://www.firebase.com/docs/web 的解决方案/guide/structuring-data.html和https://firebase.google.com/docs/database/web/读写

I have been able to make it work like this: solution from - https://www.firebase.com/docs/web/guide/structuring-data.html and https://firebase.google.com/docs/database/web/read-and-write

 comments: TaskComment[] = [];

 onGetComments(){
    var ref = firebase.database().ref('/');

    ref.child('comments/').on('child_added', (snapshot)=>{
      let userId = snapshot.val().userId;
      ref.child('users/' + userId).on('value', (user)=>{
        this.comments.push( new TaskComment( snapshot.val(), user.val() ));
      });
    });
  }

但我想将其转换为 Observable，因为这样我无法在不刷新页面的情况下查看评论是否已被删除.

but I want to convert this to Observable, because with this I can not see if the comment have been deleted without refreshing the page.

更新 1

在下面评论的帮助下，我提出了这个实现.

With the help from comment bellow I came out with this implementation.

onGetComments(){
  this.angularFireDatabase.list("/comments").valueChanges()
    .mergeMap((comments) => {
      return comments.map((comment)=>{
        this.firebaseService
          .onListData('/users', ref => ref.orderByChild('userId').equalTo(comment.userId))
          .valueChanges()
          .subscribe((user: User[])=> {
            comment.user = user[0];
          })
        return comment;
      })
    })
    .subscribe((comment)=> {
      console.log(comment);
    });
}

这将返回单独的评论，我宁愿在其中接收 Comment[]，我将尝试使用子事件:child_ added"、child_changed"、child_removed"和child_moved"与 snapshotChanges() 代替 .valueChanges().

This returns separate comments, where I would rather receive Comment[], I'll try to use child events: "child_added", "child_changed", "child_removed", and "child_moved" with snapshotChanges() instead .valueChanges().

推荐答案

好吧，根据你的更新，我个人会先创建几个辅助接口:

Ok so according to your updates, I would personally first create a couple helper interfaces:

interface User {
    userId: string;
    userName: string;
}

interface FullComment {
    commentId: string;
    userId: string;
    user: User;
}

interface CommentObject {
    commentId: string;
    commentText: string;
    userId: string;
}

然后是超级方便的辅助方法:

And then super handy helper methods:

getUser(uid: string): Observable<User> {
    return this.db.object<User>(`/users/${uid}`)
    .valueChanges()
}

getFullComment(commentObject: CommentObject): Observable<FullComment> {
    return this.getUser(commentObject.userId)
    .map((user: User) => {
        return {
            commentId: commentObject.commentId,
            commentText: commentObject.commentText,
            user: user,
        };
    });
}

最后看看获取 FullComment 对象是多么容易观察:

So finally look how easy it becomes to get the FullComment objects observable:

getComments(): Observable<FullComment[]> {
    return this.db
    .list(`/comments`)
    .valueChanges()
    .switchMap((commentObjects: CommentObject[]) => {
        // The combineLatest will convert it into one Observable
        // that emits an array like: [ [fullComment1], [fullComment2] ]
        return Observable.combineLatest(commentObjects.map(this.getFullComment));
    });
}

我认为这就是您所需要的.请让我知道这是否有帮助.使用 observable 进行愉快的编码 ;)

I think this is what you need. Please let me know if this is helpful. Happy coding with observables ;)

最新更新:之前忘记做最后一次转换来修复TypeError，所以现在应该没问题了.

Latest update: Previously forgot to make a last transformation to fix the TypeError, so now it must be ok.

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