Firestore查询单个文档 [英] Firestore query single document

问题描述

在flutter应用中，仅当最后更新的字段大于本地最后更新的字段时，才需要从firestore中拉出用户文档.

In a flutter app, I need to pull the users document from firestore only if the last updated field is greater then the last updated locally.

目前这是我要做的:

    QuerySnapshot usersData = await Firestore.instance
        .collection("users")
        .where("lastUpdated", isGreaterThan: lastUpdatedLocally)
        .where("userId", isEqualTo: userId)
        .getDocuments();
    Map<String, dynamic> userData =
        usersData.documents.isEmpty ? {} : usersData.documents.first.data;

我有三个问题:

1. 是否可以在单个文档的 get 查询中执行类似的操作，而知道文档ID是用户ID，查询整个集合似乎并不正确.
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2. 如果没有，我们是否可以通过文档ID进行查询，例如: .where(< documentId> ;, isEqualTo:userId)，而不是在名为"userId"的文档中进行归档;.
3. 最后，执行我在此所做的然后再查询单个文档(如果可能)的速度是否较慢.

1. Is it possible to do something like this in a get query for a single document, it doesn't seem right to query the whole collection knowing that the document id is the user id.
2. If not, can we do a query by the document id, something like: .where(<documentId>, isEqualTo: userId) instead of having a filed in the document called "userId".
3. And lastly, is it slower to do what I did here then to query a single document (if it is possible).

推荐答案

如果您知道文档ID，则可以肯定地 DocumentReference 而不是构建查询.

If you know the document ID, you can certainly get() a single document using a DocumentReference instead of building a Query.

DocumentSnapshot snapshot = await Firestore.instance.collection("users").document(userId).get()

性能差异可以忽略不计.Firestore查询全部基于检索到的文档总数而不是集合中的文档数执行.像上面的文档get()本质上与使用

The performance difference is negligible. Firestore queries all perform based on the total number of documents retrieved, not the number of documents in the collection. A document get() like above is essentially the same as if you were to do a query on the document ID field using FieldPath.documentId as the name of the field.

如果仅在特定字段匹配的情况下尝试获取文档，则必须使用 FieldPath.documentId 进行完整查询.但是，即使是不匹配任何文档的查询，仍然会花费您一读.

If you're trying to get the document only if a certain field matches, you will have to do a full query with FieldPath.documentId. However, even a query that doesn't match any documents will still cost you a read.

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