# 视图 *3.4 版本新功能* 本页索引 * [创建视图](https://docs.mongodb.com/manual/core/views/#%E5%88%9B%E5%BB%BA%E8%A7%86%E5%9B%BE) * [表现](https://docs.mongodb.com/manual/core/views/#%E8%A1%A8%E7%8E%B0) * [删除视图](https://docs.mongodb.com/manual/core/views/#%E5%88%A0%E9%99%A4%E8%A7%86%E5%9B%BE) * [修改视图](https://docs.mongodb.com/manual/core/views/#%E4%BF%AE%E6%94%B9%E8%A7%86%E5%9B%BE) * [支持操作](https://docs.mongodb.com/manual/core/views/#%E6%94%AF%E6%8C%81%E6%93%8D%E4%BD%9C) 从 3.4 开始, MongoDB 添加了基于已存在的集合或者 View (视图) 创建只读的 View 支持. ## 创建视图 创建或者定义一个视图, MongoDB 3.4 的介绍是: * the `viewOn` and `pipeline`options to the existing[`create`](https://docs.mongodb.com/manual/reference/command/create/#dbcmd.create)command (and[`db.createCollection`](https://docs.mongodb.com/manual/reference/method/db.createCollection/#db.createCollection)helper): ``` db.runCommand( { create: , viewOn: , pipeline: } ) ``` or if specifying a default[collation](https://docs.mongodb.com/manual/release-notes/3.4/#relnotes-collation)for the view: ``` db.runCommand( { create: , viewOn: , pipeline: , collation: } ) ``` * a new[`mongo`](https://docs.mongodb.com/manual/reference/program/mongo/#bin.mongo)shell helper[`db.createView()`](https://docs.mongodb.com/manual/reference/method/db.createView/#db.createView): ``` db.createView(, , , ) ``` ## 表现 视图具备以下几种表现: ### 只读 视图是只读的; 通过视图进行写操作会报错. 以下为支持视图的读操作: * [`db.collection.find()`](https://docs.mongodb.com/manual/reference/method/db.collection.find/#db.collection.find) * [`db.collection.findOne()`](https://docs.mongodb.com/manual/reference/method/db.collection.findOne/#db.collection.findOne) * [`db.collection.aggregate()`](https://docs.mongodb.com/manual/reference/method/db.collection.aggregate/#db.collection.aggregate) * [`db.collection.count()`](https://docs.mongodb.com/manual/reference/method/db.collection.count/#db.collection.count) * [`db.collection.distinct()`](https://docs.mongodb.com/manual/reference/method/db.collection.distinct/#db.collection.distinct) ### 索引使用 & 排序操作 * 视图使用其上游集合的索引. * 由于索引是基于集合的, 所以你不能基于视图创建, 删除或重建索引, 也不能获取视图的索引列表. * 你不能指定 [`$natural`](https://docs.mongodb.com/manual/reference/operator/meta/natural/#metaOp._S_natural) 排序. 例如, 下列操作是 *错误的*: ``` db.view.find().sort({$natural: 1}) ``` ### Project 限制 视图上的 [`find()`](https://docs.mongodb.com/manual/reference/method/db.collection.find/#db.collection.find) 方法不支持如下[projection](https://docs.mongodb.com/manual/reference/operator/projection/) 操作: * [`$`](https://docs.mongodb.com/manual/reference/operator/projection/positional/#proj._S_) * [`$elemMatch`](https://docs.mongodb.com/manual/reference/operator/projection/elemMatch/#proj._S_elemMatch) * [`$slice`](https://docs.mongodb.com/manual/reference/operator/projection/slice/#proj._S_slice) * [`$meta`](https://docs.mongodb.com/manual/reference/operator/projection/meta/#proj._S_meta) ### 不能改变名称 你不能重命名[视图](https://docs.mongoing.com/mongo-introduction/databases-and-collections/views). ### 视图创建 * 视图是在读操作期间根据需要实时计算的, 同时 MongoDB 基于视图的读操作是底层聚合管道 (aggregation pipeline) 的一部分. 因此, 视图不支持一下操作: * [`db.collection.mapReduce()`](https://docs.mongodb.com/manual/reference/method/db.collection.mapReduce/#db.collection.mapReduce), * [`$text`](https://docs.mongodb.com/manual/reference/operator/query/text/#op._S_text) 操作, 因为 `$text` 只在聚合的第一阶段有效, * [`geoNear`](https://docs.mongodb.com/manual/reference/command/geoNear/#dbcmd.geoNear) 命令和 [`$geoNear`](https://docs.mongodb.com/manual/reference/operator/aggregation/geoNear/#pipe._S_geoNear) 管道阶段. * 如果用于创建视图的聚合管道屏蔽了 `_id` 字段, 那么视图中的文档也会没有 `_id` 字段. ### 分片视图 如果视图依赖的集合是分片的, 那么视图也视为分片的. 因此, 你不能指定分片视图中 [`$lookup`](https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/#pipe._S_lookup) 的 `from`字段与 [`$graphLookup`](https://docs.mongodb.com/manual/reference/operator/aggregation/graphLookup/#pipe._S_graphLookup) 操作. ### Views 与 Collation * You can specify a default [collation](https://docs.mongodb.com/manual/reference/collation/) for a view at creation time. If no collation is specified, the view’s default collation is the “simple” binary comparison collator. That is, the view does not inherit the collection’s default collation. * String comparisons on the view use the view’s default collation. An operation that attempts to change or override a view’s default collation will fail with an error. * If creating a view from another view, you cannot specify a collation that differs from the source view’s collation. * If performing an aggregation that involves multiple views, such as with [`$lookup`](https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/#pipe._S_lookup) or [`$graphLookup`](https://docs.mongodb.com/manual/reference/operator/aggregation/graphLookup/#pipe._S_graphLookup) , the views must have the same [collation](https://docs.mongodb.com/manual/reference/collation/). ### 公共视图定义 列出集合的操作, 如 [`db.getCollectionInfos()`](https://docs.mongodb.com/manual/reference/method/db.getCollectionInfos/#db.getCollectionInfos) 和 [`db.getCollectionNames()`](https://docs.mongodb.com/manual/reference/method/db.getCollectionNames/#db.getCollectionNames), 的结果中会包括它们的视图. > IMPORTANT > > 视图定义是公开的; 即在视图上的 [`db.getCollectionInfos()`](https://docs.mongodb.com/manual/reference/method/db.getCollectionInfos/#db.getCollectionInfos) 和 `explain` 操作将会包括定义视图的管道. 因此, 请避免直接引用视图定义中敏感的字段和值. ## 删除视图 要删除视图, 请使用视图上的 [`db.collection.drop()`](https://docs.mongodb.com/manual/reference/method/db.collection.drop/#db.collection.drop) 方法. ## 修改视图 你可以通过删除或者重建的方式修改视图, 也可以使用 [`collMod`](https://docs.mongodb.com/manual/reference/command/collMod/#dbcmd.collMod) 命令. ## 支持操作 以下操作支持视图, 除了本文中提到的限制除外: | 命令 | 方法 | | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | [`create`](https://docs.mongodb.com/manual/reference/command/create/#dbcmd.create) | [`db.createCollection()`](https://docs.mongodb.com/manual/reference/method/db.createCollection/#db.createCollection) [`db.createView()`](https://docs.mongodb.com/manual/reference/method/db.createView/#db.createView) | | [`collMod`](https://docs.mongodb.com/manual/reference/command/collMod/#dbcmd.collMod) | | | | [`db.getCollection()`](https://docs.mongodb.com/manual/reference/method/db.getCollection/#db.getCollection) [`db.getCollectionInfos()`](https://docs.mongodb.com/manual/reference/method/db.getCollectionInfos/#db.getCollectionInfos) [`db.getCollectionNames()`](https://docs.mongodb.com/manual/reference/method/db.getCollectionNames/#db.getCollectionNames) | | [`find`](https://docs.mongodb.com/manual/reference/command/find/#dbcmd.find) [`distinct`](https://docs.mongodb.com/manual/reference/command/distinct/#dbcmd.distinct) [`count`](https://docs.mongodb.com/manual/reference/command/count/#dbcmd.count) | [`db.collection.aggregate()`](https://docs.mongodb.com/manual/reference/method/db.collection.aggregate/#db.collection.aggregate) [`db.collection.find()`](https://docs.mongodb.com/manual/reference/method/db.collection.find/#db.collection.find) [`db.collection.findOne()`](https://docs.mongodb.com/manual/reference/method/db.collection.findOne/#db.collection.findOne) [`db.collection.count()`](https://docs.mongodb.com/manual/reference/method/db.collection.count/#db.collection.count) [`db.collection.distinct()`](https://docs.mongodb.com/manual/reference/method/db.collection.distinct/#db.collection.distinct) | 原文链接： 译者 ：王恒