替换为（转换阶段）

替换为（转换阶段）

说明

将现有文档中的字段替换为给定表达式中的字段。

该表达式可以是字面量映射，也可以是计算结果为映射的表达式。如果给定表达式的计算结果不是映射，此阶段会返回错误。

支持 3 种模式：

• full_replace：将整个文档替换为给定表达式的结果，并省略其中未显示的字段。
• merge_overwrite_existing：将表达式与现有文档合并，并使用表达式中的值覆盖文档中的字段值
• merge_keep_existing：将表达式与现有文档合并，仅在文档中不存在字段值时添加字段值。

示例

创建一个包含以下文档的 cities 集合：

Node.js

await db.collection("cities").doc("SF").set({name: "San Francisco", population: 800000, location: {country: "USA", state: "California"}});
await db.collection("cities").doc("TO").set({name: "Toronto", population:  3000000, province: "ON", location: {country: "Canada", province: "Ontario"}});
await db.collection("cities").doc("NY").set({name: "New York", location: {country: "USA", state: "New York"}});
await db.collection("cities").cov("AT").set({name: "Atlantis", population: null});

使用 full_replace 模式获取修改后的文档版本

提取嵌套的 location 字段，舍弃所有其他数据：

Node.js

  const names = await db.pipeline()
    .collection("/cities")
    .replace_with(Field.of("location"), "full_replace")
    .execute();
    

Java

Pipeline.Snapshot names =
    firestore.pipeline().collection("cities").replaceWith(field("location")).execute().get();
PipelineSnippets.java

这会生成以下文档：

{ country: "USA", state: "California" },
{ country: "Canada", province: "Ontario" },
{ country: "USA", state: "New York" },
{ }

使用 merge_overwrite_existing 模式设置字段

将所有文档的 population 字段设置为 0，覆盖现有值：

Node.js

  const censoredResults = await db.pipeline()
    .collection("/cities")
    .replace_with(map("population", 0), "merge_overwrite_existing")
    .execute();
    

这会生成以下文档：

{name: "San Francisco", population: 0, location: {country: "USA", state: "California"}},
{name: "Toronto", population: 0, province: "ON", location: {country: "Canada", province: "Ontario"}},
{name: "New York", population: 0, location: {country: "USA", state: "New York"}},
{name: "Atlantis", population: 0}

使用 merge_keep_existing 模式添加默认值

当文档中不存在 location 字段时，为其设置默认值：

Node.js

  const defaultedResults = await db.pipeline()
    .collection("/cities")
    .replace_with(map("location", "unknown"), "merge_keep_existing")
    .execute();
    

这会生成以下文档：

{ name: "San Francisco", population: 800000, location: { country: "USA", state: "California" } },
{ name: "Toronto", province: "ON", population: 3000000, location: { country: "Canada", province: "Ontario" } },
{ name: "New York", location: { country: "USA", state: "New York" } },
{ name: "Atlantis", population: null, location: "unknown" }