MongoDB: Comparing 2 fields in the same document

Introduction

MongoDB is a popular NoSQL database known for its flexibility and performance. Often in database operations, there is a need to compare values of different fields within the same document. This can be required for a variety of reasons such as validation checks, data cleanup, analytics, and more. In this tutorial, we’ll explore different ways to compare two fields in the same document in MongoDB using the MongoDB Query Language (MQL).

Basics of Field Comparison

In MongoDB, comparing two fields can be done directly in a find query using the $expr operator, which allows the use of aggregation expressions within the query language. Let’s start with a basic example:

db.collection.find({ $expr: { $eq: ["$field1", "$field2"] } })

This query will find all documents in the collection where the value of field1 is equal to the value of field2.

Using the $expr Operator

The $expr operator is very powerful and can be used with other aggregation operators for more complex comparison such as greater-than, less-than, etc. Here’s an example comparing numerical values:

db.collection.find({ $expr: { $gt: ["$field1", "$field2"] } })

This query will return documents where the value of field1 is greater than field2.

Combining Conditions

Multiple conditions can be combined using logical operators like $and and $or. Check this example:

db.collection.find({
  $expr: {
    $and: [
      { $gt: ["$field1", "$field2"] },
      { $lt: ["$field1", $value] }
    ]
  }
})

This retrieves documents where field1 is greater than field2 and also less than some other specified value.

Complex Comparisons Using Aggregation Pipeline

For more advanced comparisons, you can use the aggregation pipeline. Let’s look into a scenario where you want to compare the lengths of two different array fields:

db.collection.aggregate([
  {
    $project: {
      field1: 1,
      field2: 1,
      fieldComparison: { $eq: [{ $size: "$field1" }, { $size: "$field2" }] }
    }
  },
  {$match: {fieldComparison: true}}
])

In this example, the $project stage adds a new field fieldComparison, which holds the result of the comparison between the sizes of field1 and field2. The $match stage then filters the documents to only those where fieldComparison is true.

Application in Real-World Scenarios

Beyond simple comparisons, real-world database operations often require checking conditional logic across fields. Here is how you can handle some typical cases:

Checking for Inconsistencies

Suppose you have two date fields and need to find documents where one date is earlier than the other:

db.collection.find({ $expr: { $lt: ["$startDate", "$endDate"] } })

This can help identify potential data inconsistencies or errors where a start date is erroneously set after the end date.

Conditional Updates

Using the $expr operator, you can also perform conditional updates. The following operation increments field1 by 10 if it is initially less than field2:

db.collection.update({ $expr: { $lt: ["$field1", "$field2"] } },
  { $inc: {field1: 10} }
)

Monitoring Data Integrity

Field comparison is used for data validation rules to sustain data integrity in applications. You need to enforce that a certain numerical field quantityAvailable must always be less or equal to another field quantityInStock:

db.collection.find({
  $expr: { $gte: ["$quantityInStock", "$quantityAvailable"] }
})

This query ensures that the available quantity isn’t mistakenly inputted as more than what is in stock.

Conclusion

In Mongo’s expressive query language, comparing fields within the same document can range from simple equality checks to complex, conditional logic, laying the groundwork for robust data processing and validation. Using $expr in find and update operations as well as in aggregation pipelines offers a flexible toolkit for MongoDB practitioners.