C100DEV MongoDB Overview and Document Model Practice Question

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✗

  db.sensors.find( { $where: "this.readings.temperature > 30" } )

  Why it's wrong here

  The $where operator allows JavaScript expressions, but it is inefficient, deprecated for most use cases, and can pose security risks. While it could technically work, it is not the recommended or correct approach for querying nested fields. The standard dot notation is preferred.

• ✓

  db.sensors.find( { "readings.temperature": { $gt: 30 } } )

  Why this is correct

  This query uses dot notation to access the nested field 'temperature' within the 'readings' object and applies the $gt operator to find values greater than 30. Dot notation is the correct way to query nested fields in MongoDB, making this the accurate query for the scenario.

• ✗

  db.sensors.find( { readings: { temperature: { $gt: 30 } } } )

  Why it's wrong here

  This query attempts to match an exact subdocument where the 'readings' field equals an object with a 'temperature' field containing a query operator. In MongoDB, this would look for a document where 'readings' is exactly that object, not a nested field comparison. It is incorrect for querying a nested value.

• ✗

  db.sensors.find( { "readings": { $elemMatch: { temperature: { $gt: 30 } } } } )

  Why it's wrong here

  $elemMatch is used to match elements within an array, but 'readings' is a nested object, not an array. Using $elemMatch here would not correctly match the nested field and could lead to errors or no results. Thus, this query is incorrect for the given document structure.