Example of a MongoDB Query that uses $elemMatch

Snowpal API: When do you need to use $elemMatch in a MongoDB query?

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A simple example to illustrate MongoDB’s $elemMatch. In the code below, we fetch courses a teacher has access to.

There are 3 arguments to this method:

1. user: A user object so we can return courses this particular user has access to.

2. acl: What level of access should this teacher have?

3. archived: Return archived or unarchived courses as dictated by this boolean.

Could the query have looked like this instead?

Let’s try removing the use of $elemMatch and make the query simpler.

        where :_type.in => [PlanitApp::PrivateTeacherCourse, PlanitApp::SharedTeacherCourse],
          :archived => archived,
          :"authorization.users.id" => user.id,
          :"authorization.users.acl" => acl

Will this work and return what we expect? Not quite.

Our intent is to return only those courses where the given user (user.id) has a certain level of access (acl) but authorization.users is a nested array of users with multiple attributes (with ID and ACL being two of those). Though we want both those conditions to be applied within that nested array in the document, not using $elemMatch will not scope the results to only those courses as it will instead apply each of those conditions separately across different documents in that collection thereby returning incorrect results.

So, when you use nested arrays in a NoSQL document, be wary of this.

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