db.createView()¶

Read Only只读¶

Views are read-only; write operations on views will error.视图是只读的；对视图的写入操作将出错。

The following read operations can support views:以下读取操作可以支持视图：

• db.collection.find()
• db.collection.findOne()
• db.collection.aggregate()
• db.collection.countDocuments()
• db.collection.estimatedDocumentCount()
• db.collection.count()
• db.collection.distinct()

Index Use and Sort Operations索引使用和排序操作¶

• Views use the indexes of the underlying collection.视图使用基础集合的索引。
• As the indexes are on the underlying collection, you cannot create, drop or re-build indexes on the view directly nor get a list of indexes on the view.由于索引位于基础集合上，因此无法直接在视图上创建、删除或重新构建索引，也无法获取视图上的索引列表。
• Starting in MongoDB 4.4, you can specify a $natural sort when running a find command on a view. 从MongoDB 4.4开始，在视图上运行find命令时，可以指定$natural排序。Prior versions of MongoDB do not support $natural sort on views.MongoDB的早期版本不支持对视图进行$natural排序。
• The view’s underlying aggregation pipeline is subject to the 100 megabyte memory limit for blocking sort and blocking group operations. 视图的底层聚合管道受阻止排序和阻止组操作的100 MB内存限制的限制。Starting in MongoDB 4.4, you can issue a find command with allowDiskUse: true on the view to allow MongoDB to use temporary files for blocking sort and group operations.从MongoDB 4.4开始，您可以在视图上发出一个带有allowDiskUse:true的find命令，以允许MongoDB使用临时文件来阻止排序和分组操作。

  Prior to MongoDB 4.4, only the aggregate command accepted the allowDiskUse option.在MongoDB 4.4之前，只有aggregate命令接受allowDiskUse选项。

  See also参阅

  For more information on blocking sort operation memory limits, see Sort Operations.有关阻止排序操作内存限制的更多信息，请参阅排序操作。

Projection Restrictions投影限制¶

find() operations on views do not support the following projection operators:视图上的find()操作不支持以下projection运算符：

• $
• $elemMatch
• $slice
• $meta

Immutable Name不可变名称¶

You cannot rename views.不能重命名视图。

View Creation视图创建¶

• Views are computed on demand during read operations, and MongoDB executes read operations on views as part of the underlying aggregation pipeline. 视图是在读取操作期间按需计算的，MongoDB作为底层聚合管道的一部分对视图执行读取操作。As such, views do not support operations such as:因此，视图不支持以下操作：
  • db.collection.mapReduce(),
  • $text operator, since $text operation in aggregation is valid only for the first stage,$text运算符，因为聚合中的$text操作仅对第一阶段有效，
  • $geoNear pipeline stage.
• If the aggregation pipeline used to create the view suppresses the _id field, documents in the view do not have the _id field.如果用于创建视图的聚合管道不包含_id字段，则视图中的文档没有_id字段。

Sharded View切分视图¶

Views are considered sharded if their underlying collection is sharded. 如果视图的基础集合是分片的，则认为视图是分片的。As such, you cannot specify a sharded view for the from field in $lookup and $graphLookup operations.因此，不能在from field in $lookup和$graphLookup操作中为from字段指定分片视图。

Views and Collation视图和排序¶

• You can specify a default collation for a view at creation time. 可以在创建时为视图指定默认排序规则。If no collation is specified, the view’s default collation is the “simple” binary comparison collator. 如果未指定排序规则，则视图的默认排序规则为“简单”二进制比较排序规则。That is, the view does not inherit the collection’s default collation.也就是说，视图不会继承集合的默认排序规则。
• String comparisons on the view use the view’s default collation. 视图上的字符串比较使用视图的默认排序规则。An operation that attempts to change or override a view’s default collation will fail with an error.尝试更改或覆盖视图默认排序规则的操作将失败并出现错误。
• If creating a view from another view, you cannot specify a collation that differs from the source view’s collation.如果从其他视图创建视图，则不能指定与源视图排序规则不同的排序规则。
• If performing an aggregation that involves multiple views, such as with $lookup or $graphLookup, the views must have the same collation.如果执行涉及多个视图的聚合，例如使用$lookup或$graphLookup，则这些视图必须具有相同的排序规则。

Resource Locking资源锁定¶

db.createView() obtains an exclusive lock on the specified collection or view for the duration of the operation. 在操作期间获取指定集合或视图的独占锁。All subsequent operations on the collection must wait until db.createView() releases the lock. 集合上的所有后续操作都必须等待db.createView()释放锁。db.createView() typically holds this lock for a short time.db.createView()通常会在短时间内持有此锁。

Creating a view requires obtaining an additional exclusive lock on the system.views collection in the database. 创建视图需要在数据库中的system.views集合上获得额外的独占锁。This lock blocks creation or modification of views in the database until the command completes.此锁将阻止在数据库中创建或修改视图，直到命令完成。

Prior to MongoDB 4.2, db.createView() obtained an exclusive lock on the parent database, blocking all operations on the database and all its collections until the operation completed.在MongoDB 4.2之前，db.createView()获得了父数据库的独占锁，阻止了数据库及其所有集合上的所有操作，直到操作完成。

Create a View from a Single Collection从单个集合创建视图¶

Given a collection survey with the following documents:提供了包含以下文件的集合survey：

{ _id: 1, empNumber: "abc123", feedback: { management: 3, environment: 3 }, department: "A" }
{ _id: 2, empNumber: "xyz987", feedback: { management: 2, environment: 3 }, department: "B" }
{ _id: 3, empNumber: "ijk555", feedback: { management: 3, environment: 4 }, department: "A" }

The following operation creates a managementFeedback view with the _id, feedback.management, and department fields:以下操作使用_id、feedback.management和department字段创建managementFeedback视图：

db.createView(
   "managementFeedback",
   "survey",
   [ { $project: { "management": "$feedback.management", department: 1 } } ]
)

db.managementFeedback.find()

{ "_id" : 1, "department" : "A", "management" : 3 }
{ "_id" : 2, "department" : "B", "management" : 2 }
{ "_id" : 3, "department" : "A", "management" : 3 }

db.managementFeedback.aggregate([ { $sortByCount: "$department" } ] )

{ "_id" : "A", "count" : 2 }
{ "_id" : "B", "count" : 1 }

Create a View from Multiple Collections从多个集合创建视图¶

Given the following two collections:鉴于以下两个集合：

• The orders collection:orders集合：

  { "_id" : 1, "item" : "abc", "price" : NumberDecimal("12.00"), "quantity" : 2 }
  { "_id" : 2, "item" : "jkl", "price" : NumberDecimal("20.00"), "quantity" : 1 }
  { "_id" : 3, "item" : "abc", "price" : NumberDecimal("10.95"), "quantity" : 5 }
  { "_id" : 4, "item" : "xyz", "price" : NumberDecimal("5.95"), "quantity" : 5 }
  { "_id" : 5, "item" : "xyz", "price" : NumberDecimal("5.95"), "quantity" : 10 }

• The inventory collection:inventory收集：

  { "_id" : 1, "sku" : "abc", description: "product 1", "instock" : 120 }
  { "_id" : 2, "sku" : "def", description: "product 2", "instock" : 80 }
  { "_id" : 3, "sku" : "ijk", description: "product 3", "instock" : 60 }
  { "_id" : 4, "sku" : "jkl", description: "product 4", "instock" : 70 }
  { "_id" : 5, "sku" : "xyz", description: "product 5", "instock" : 200 }

The following db.createView() example specifies a $lookup stage to create a view from the join of the two collections:以下db.createView()示例指定一个$lookup阶段，用于从两个集合的联接创建视图：

db.createView (
   "orderDetails",
   "orders",
   [
     { $lookup: { from: "inventory", localField: "item", foreignField: "sku", as: "inventory_docs" } },
     { $project: { "inventory_docs._id": 0, "inventory_docs.sku": 0 } }
   ]
)

db.orderDetails.find()

{
   "_id" : 1,
   "item" : "abc",
   "price" : NumberDecimal("12.00"),
   "quantity" : 2,
   "inventory_docs" : [ { "description" : "product 1", "instock" : 120 } ]
}
{
   "_id" : 2,
   "item" : "jkl",
   "price" : NumberDecimal("20.00"),
   "quantity" : 1,
   "inventory_docs" : [ { "description" : "product 4", "instock" : 70 } ]
}
{
   "_id" : 3,
   "item" : "abc",
   "price" : NumberDecimal("10.95"),
   "quantity" : 5,
   "inventory_docs" : [ { "description" : "product 1", "instock" : 120 } ]
}
{
   "_id" : 4,
   "item" : "xyz",
   "price" : NumberDecimal("5.95"),
   "quantity" : 5,
   "inventory_docs" : [ { "description" : "product 5", "instock" : 200 } ]
}
{
   "_id" : 5,
   "item" : "xyz",
   "price" : NumberDecimal("5.95"),
   "quantity" : 10,
   "inventory_docs" : [ { "description" : "product 5", "instock" : 200 } ]
}

db.orderDetails.aggregate( [ { $sortByCount: "$item" } ] )

{ "_id" : "xyz", "count" : 2 }
{ "_id" : "abc", "count" : 2 }
{ "_id" : "jkl", "count" : 1 }

Create a View with Default Collation创建具有默认排序规则的视图¶

Given the places collection with the following document:给定带有以下文档的places集合：

{ _id: 1, category: "café" }
{ _id: 2, category: "cafe" }
{ _id: 3, category: "cafE" }

The following operation creates a view, specifying collation at the view level:以下操作创建视图，并在视图级别指定排序规则：

db.createView(
   "placesView",
   "places",
   [ { $project: { category: 1 } } ],
   { collation: { locale: "fr", strength: 1 } }
)

String comparisons on the view use the view’s default collation. 视图上的字符串比较使用视图的默认排序规则。For example, the following operation uses the view’s collation:例如，以下操作使用视图的排序规则：

db.placesView.count( { category: "cafe" } )

The operation returns 3.操作返回3。

An operation that attempts to change or override a view’s default collation will fail with an error.尝试更改或覆盖视图默认排序规则的操作将失败并出现错误。