MongoDB: Make all documents have the same structure by filling missing fields

Introduction

In MongoDB, documents in a collection do not need to have the same set of fields or structure. This flexible schema is advantageous in certain applications but can present challenges when performing queries, aggregations, or when exporting data to systems that require a consistent format. In this tutorial, we will explore how to enforce a uniform document structure by adding missing fields to documents within a MongoDB collection. We will cover a variety of techniques ranging from basic updates to advanced aggregation pipeline stages, complete with code examples and expected outputs.

Understanding MongoDB Schemas

Before diving into code, it’s critical to understand that MongoDB is a schema-less database, which means it does not require a pre-defined schema before inserting data. A collection can have documents that are entirely different from one another. For example:

{ '_id': 1, 'name': 'Alice', 'age': 30 }
{ '_id': 2, 'name': 'Bob' }

The second document is missing the ‘age’ field. Let’s see how we can ensure that all documents have the same fields, including those that are initially absent.

Using the update() Method

The update() method in MongoDB is one of the simplest ways to add fields to documents that miss them. The method has several forms, such as updateOne, updateMany, and the deprecated update. You can specify a query that selects the documents and then the fields to add or update.

Example 1: Adding a Single Missing Field

db.collection.updateMany(
  { 'age': { '$exists': false } },
  { '$set': { 'age': null } }
);

This code will add the ‘age’ field with a null value for all documents where the ‘age’ field does not exist. All documents in the collection will now have an ‘age’ field.

Example 2: Adding Multiple Missing Fields

db.collection.updateMany(
  { '$or': [
    { 'age': { '$exists': false } },
    { 'email': { '$exists': false } }
  ] },
  { '$set': { 'age': null, 'email': null } }
);

If a document is missing either ‘age’ or ’email’ fields, this command will add both with null values.

Using the Aggregation Framework

For more complex tasks, such as adding default values based on other fields, we can leverage the Aggregation Framework. The $addFields or $set stage introduced in MongoDB 3.4 can add new fields or update existing ones with an aggregation pipeline.

Example 3: Adding Fields with Conditional Logic

db.collection.aggregate([
  {
    '$set': {
      'age': { '$ifNull': ['$age', 20] }, // Sets a default age if missing
      'email': { '$ifNull': ['$email', ''] }, // Sets an empty string if email is absent
      'status': 'active'
    }
  }
]);

This pipeline will output the modified documents, where missing ‘age’ and ’email’ fields are set to default values, and all documents receive a new ‘status’ field with the value ‘active’.

Example 4: Adding Missing Fields in Bulk Operations

Bulk write operations can be efficient when working with a large number of documents. Here’s how:

const bulkOps = db.collection.initializeOrderedBulkOp();
db.collection.find().forEach(doc => {
  const update = { '$set': {} };
  if (typeof doc.age === 'undefined') { update['$set']['age'] = null; }
  if (typeof doc.email === 'undefined') { update['$set']['email'] = ''; }
  if (Object.keys(update['$set']).length > 0) {
    bulkOps.find({ '_id': doc._id }).updateOne(update);
  }
});
bulkOps.execute();

We initialize a bulk operation, iterate over each document, preparing the updates, and then execute the bulk operation for optimized performance.

Automating Field Addition with MongoDB Triggers

If you need to ensure that documents are always inserted with a specific set of fields, you might consider using database triggers, especially in a MongoDB Atlas environment which supports triggers natively.

Example 5: Creating a Trigger for Document Insertion

In MongoDB Atlas, you can create a trigger that listens to the insert event on a collection and adds missing fields automatically:

// Function to run upon an insertion event
exports = function(changeEvent) {
  const fullDocument = changeEvent.fullDocument;
  const defaultFields = { 'age': null, 'email': '', 'status': 'active' };
  let updateRequired = false;

  Object.keys(defaultFields).forEach(key => {
    if (fullDocument[key] === undefined) {
      fullDocument[key] = defaultFields[key];
      updateRequired = true;
    }
  });

  if (updateRequired) {
    const collection = context.services.get('mongodb-atlas').db('your_db').collection('your_collection');
    collection.updateOne({ '_id': fullDocument._id }, { '$set': fullDocument });
  }
};

The trigger function adds default values for missing fields whenever a new document is inserted.

Guarding Schema Constraints with JSON Schema Validation

MongoDB 3.6 introduced JSON schema validation that provides a way to enforce document structures during updates and insertions:

Example 6: Enforcing Field Presence with JSON Schema

db.runCommand({
  'collMod': 'collection',
  'validator': { '$jsonSchema': {
    'bsonType': 'object',
    'required': ['name', 'age', 'email', 'status'],
    'properties': {
      'name': {
        'bsonType': 'string',
        'description': 'must be a string and is required'
      },
      'age': {
        'bsonType': ['int', 'null'],
        'description': 'must be an integer or null and is required'
      },
      // Define similar schemas for other fields...
    }
  } },
  'validationLevel': 'strict',
  'validationAction': 'error'
});

With JSON schema validation, MongoDB enforces that every document in the ‘collection’ contains the ‘name’, ‘age’, ’email’, and ‘status’ fields on all inserts and updates, otherwise, the operation fails.

Conclusion

Standardizing document structure in a MongoDB collection is essential for data integrity, eases querying, and adapts documents for use in systems requiring consistent data formats. Techniques for enforcing uniform document structures in MongoDB range from simple field addition to complex schema validation. Throughout this tutorial, we’ve explored methods to programmatically ensure all documents contain the same fields, thus paving the way for more organized and maintainable data management practices.