Views

Read Only

Views are read-only; write operations on views will error.

The following read operations can support views:

• 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

• Views use the indexes of the underlying collection.

• 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.

• Starting in MongoDB 4.4, you can specify a $natural sort when running a find command on a view. Prior versions of MongoDB do not support $natural sort on views.

• The view's underlying aggregation pipeline is subject to the 100 megabyte memory limit for blocking sort and blocking group operations. 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.

  Prior to MongoDB 4.4, only the aggregate command accepted the allowDiskUse option.

  Tip

  For more information on blocking sort operation memory limits, see Sort Operations.

Projection Restrictions

find() operations on views do not support the following projection operators:

• $

• $elemMatch

• $slice

• $meta

Immutable Name

You cannot rename views.

View Creation

• Views are computed on demand during read operations, and MongoDB executes read operations on views as part of the underlying aggregation pipeline. As such, views do not support operations such as:

  • db.collection.mapReduce(),

  • $text operator, since $text operation in aggregation is valid only for the first stage,

  • $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.

When you query a view, the:

• Query filter, projection, sort, skip, limit, and other operations for db.collection.find() are converted to the equivalent aggregation pipeline stages.

• Converted aggregation pipeline stages are added to the end of the aggregation pipeline for the view. This does not modify the view's underlying pipeline, which is set when you create the view.

• Aggregation pipeline optimizer reshapes the view aggregation pipeline stages to improve performance. This does not change the query results.

Sharded View

Views are considered sharded if their underlying collection is sharded. As such, you cannot specify a sharded view for the from field in $lookup and $graphLookup operations.

Views and Collation

• You can specify a default 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 or $graphLookup, the views must have the same collation.

Snapshot Isolation

Views do not maintain timestamps of collection changes and do not support point-in-time or snapshot read isolation.

Public View Definition

Operations that lists collections, such as db.getCollectionInfos() and db.getCollectionNames(), include views in their outputs.

Use db.createView() to Create a View

Use db.createView() to create a view that is limited to first year students:

db.createView(
   "firstYears",
   "students",
   [ { $match: { year: 1 } } ]
)

In the example:

• firstYears is the name of the new view.

• students is the collection the view is based on.

• $match is an aggregation expression that matches first year students in the students collection.

This example queries the view:

db.firstYears.find({}, { _id: 0 } )

The following output only contains the documents with data on first year students. The { _id: 0 } projection suppresses the _id field in the output.

[
  { sID: 22001, name: 'Alex', year: 1, score: 4 },
  { sID: 22021, name: 'Drew', year: 1, score: 3.2 },
  { sID: 21022, name: 'Farmer', year: 1, score: 2.2 }
]

Use db.createCollection() to Create a View

The db.createCollection() method allows you to create a collection or a view with specific options.

The following example creates a graduateStudents view. The view only contains documents selected by the $match stage. The optional collation setting determines the sort order.

db.createCollection(
   "graduateStudents",
   {
      viewOn: "students",
      pipeline: [ { $match: { $expr: { $gt: [ "$year", 4 ] } } } ],
      collation: { locale: "en", caseFirst: "upper" }
   }
)

The following example queries the view. The $unset stage removes the _id field from the output for clarity.

db.graduateStudents.aggregate(
   [
      { $sort: { name: 1 } },
      { $unset: [ "_id" ] }
   ]
)

When the output is sorted, the $sort stage uses the collation ordering to sort uppercase letters before lowercase letters.

[
  { sID: 18020, name: 'Harley', year: 5, score: 2.8 },
  { sID: 17301, name: 'harley', year: 6, score: 3.1 }
]

Use a View to Join Two Collections

It is often convenient to use $lookup to create a view over two collections and then run queries against the view. Applications can query the view without having to construct or maintain complex pipelines.

Create two sample collections, inventory and orders:

db.inventory.insertMany( [
   { prodId: 100, price: 20, quantity: 125 },
   { prodId: 101, price: 10, quantity: 234 },
   { prodId: 102, price: 15, quantity: 432 },
   { prodId: 103, price: 17, quantity: 320 }
] )
db.orders.insertMany( [
   { orderID: 201, custid: 301, prodId: 100, numPurchased: 20 },
   { orderID: 202, custid: 302, prodId: 101, numPurchased: 10 },
   { orderID: 203, custid: 303, prodId: 102, numPurchased: 5 },
   { orderID: 204, custid: 303, prodId: 103, numPurchased: 15 },
   { orderID: 205, custid: 303, prodId: 103, numPurchased: 20 },
   { orderID: 206, custid: 302, prodId: 102, numPurchased: 1 },
   { orderID: 207, custid: 302, prodId: 101, numPurchased: 5 },
   { orderID: 208, custid: 301, prodId: 100, numPurchased: 10 },
   { orderID: 209, custid: 303, prodId: 103, numPurchased: 30 }
] )

Create a view that combines elements from each collection:

db.createView( "sales", "orders", [
   {
      $lookup:
         {
            from: "inventory",
            localField: "prodId",
            foreignField: "prodId",
            as: "inventoryDocs"
         }
   },
   {
      $project:
         {
           _id: 0,
           prodId: 1,
           orderId: 1,
           numPurchased: 1,
           price: "$inventoryDocs.price"
         }
   },
      { $unwind: "$price" }
] )

In the example:

• db.createView() creates the sales view.

• The sales view is based on the orders collection.

• The $lookup stage uses the prodId field in the orders collection to "join" documents in the inventory collection that have matching prodId fields.

• The matching documents are added as an array in the inventoryDocs field.

• The $project stage selects a subset of the available fields.

• The $unwind stage converts the price field from an array to a scalar value.

The documents in the sales view are:

{ prodId: 100, numPurchased: 20, price: 20 },
{ prodId: 101, numPurchased: 10, price: 10 },
{ prodId: 102, numPurchased: 5, price: 15 },
{ prodId: 103, numPurchased: 15, price: 17 },
{ prodId: 103, numPurchased: 20, price: 17 },
{ prodId: 102, numPurchased: 1, price: 15 },
{ prodId: 101, numPurchased: 5, price: 10 },
{ prodId: 100, numPurchased: 10, price: 20 },
{ prodId: 103, numPurchased: 30, price: 17 }

To find the total amount sold of each product, query the view:

db.sales.aggregate( [
   {
      $group:
         {
            _id: "$prodId",
            amountSold: { $sum: { $multiply: [ "$price", "$numPurchased" ] } }
         }
   }
] )

The output is:

[
  { _id: 100, amountSold: 600 },
  { _id: 103, amountSold: 1105 },
  { _id: 101, amountSold: 150 },
  { _id: 102, amountSold: 90 }
]