Filter Incoming Queries

Overview

A filter modifies an incoming MongoDB query to return only a subset of the results matched by the query. Adding a filter to a collection allows you to control the shape of queried documents and can improve query performance.

Filters add additional query parameters and omit fields from query results before Atlas App Services runs the query. Every filter has three components:

• An "apply when" expression that determines if the filter applies to an incoming request. You can use variables like %%user and %%request in the "apply when" expression. However, you cannot use expansions that refer to a document like %%root because App Services evaluates the "apply when" expression before reading any documents.

• An optional query expression, which merges with the existing query of any request the filter applies to.

• An optional projection document, which uses standard MongoDB projection syntax and merges with the existing projection of any request the filter applies to.

How App Services Applies Filters

App Services evaluates and applies filters for every MongoDB request where rules apply, such as:

• A query on a collection.

• A write to a document.

• The start of a Device Sync session.

Multiple filters may apply to a single request.

A filter applies to a given request if its "apply when" expression evaluates to true given that request's context. If a filter applies to a request, App Services merges the filter's query or projection into the requested operation's existing query and projection.

App Services applies filters to the request before it sends the request to MongoDB.

{ "owner_id": "%%user.id" }

Define Filters

You can use filters to optimize queries, minimize compute overhead, and secure sensitive data. Filters are most useful for cross-cutting concerns that affect some or all of your queries.

Consider using filters if you want a centralized system to:

• Restrict queries to a subset of all documents

• Omit sensitive data or unused fields