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MongoDB 3.4 Views

 | January 13, 2017 |  Posted In: Insight for DBAs, MongoDB

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MongoDB 3.4 ViewsThis blog post covers MongoDB 3.4 views, one of the more recent MongoDB features.

Views are often used in relational databases to achieve both data security and a high level of abstraction, making it easier to retrieve data. Unlike regular tables, views neither have a physical schema nor use disk space. They execute a pre-specified query. There are exceptions (such as materialized views and pre-executed views), but as a default the engine actually executes a query and then sends the result set as a single table when a view is used.

In MySQL, a simple view can be defined as:

The query above shows only the users and host field, rather than all the table fields. Anyone who queries this view sees a table that only has the user and host fields.

This feature was not available in previous MongoDB versions. All we could do was either deny reads in a collection (which would make it useless to the user) or allow reads to the entire collection (which  was pretty unsafe).

The views feature request was open for a while, and as we can see there was a considerable number of votes to make this feature available: https://jira.mongodb.org/browse/SERVER-142.

MongoDB 3.4 views are non-materialized views, and behind the scenes the engine runs an aggregation. Creating a view requires that we specify a collection or a previous existing view. When a view is the source collection from another view, it allows us to execute a chained aggregation.

To create a view, we should use the db.createView(view_name’,source,[pipeline]) command, specifying the view name, the view source collection and the aggregation pipeline. This aggregation pipeline, as well as the other parameters, is saved in the system.views collection. This is the only space that the view will use in the system. A new document is saved in the system.views collection for each view created.

Although views seem very easy to create, a few pitfalls when using them.

Since views always run an aggregation, an index is desired to cover the aggregation $match pipeline, or slow responses might be expected during the full collection scans.

Cascading aggregations (creating views of views) can be slow, as the view does not have any data and therefore cannot be indexed. MongoDB neither checks the collection fields nor the collection existence before creating the view. If there is no collection, the view returns an empty cursor.

Views appear as a collection when we are listing them. The show collections command shows us views as one collection, but such collections are read-only. To drop a view, we simply execute db.collection.drop(). The collection is removed from the system.collections, but the data remains untouched because it only removes the code that generates the view result.

How to create views:

In this step-by-step, we will create a view and restrict the user UserReadOnly to read privileges only:

1. Populate collection:

2. Create view that only shows full names:

3. Create a user-defined role that only gives access to the views:

Create a file “createviewOnlyRole.js” with the following javascript, or copy and paste the following code: 

Then authenticate and use the desired database to create this role. In our case:

4. Create a new user assigned to the readAnyView role. This new user is only able to query against views, and they must know the view name because no other privileges are granted:

Notes: If you receive an error when trying to execute the .js file, please create a new role that grants find in the system.views collection:

For more information about user-defined roles, please check please check the user-defined docs.

This should help explain MongoDB 3.4 views. Please feel free to contact me @AdamoTonete or @percona for any questions and suggestions.

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One Comment

  • Can we query views from mongo clients from node js? I tried it using nodejs mongo client and queried it like a normal collection but it doesn’t seem to work. (but gives no error either)

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