How to Access MongoDB Fields with Dots in Names

MongoDB interprets dots in field names as field separators for dot notation, which causes the issue you’re experiencing with ‘No. of items’. While bracket notation works for JavaScript objects, it doesn’t resolve MongoDB’s internal dot notation interpretation. To properly access fields with dots in their names, you need to use MongoDB’s aggregation pipeline operators or alternative approaches.

Contents

• Understanding the Problem
• Solution 1: Using Aggregation Operators
• Solution 2: Encoding Field Names
• Solution 3: Alternative Query Structure
• Best Practices
• Complete Example

Understanding the Problem

When you have a field name like ‘No. of items’, MongoDB’s dot notation parser treats the dot as a separator between nested field paths. This means mappedTaxonomies.${orgId}.attributes.${column}.value where ${column} is ‘No. of items’ gets interpreted as accessing a nested field structure rather than a single field with a dot in its name.

According to the MongoDB documentation, using field names containing dots is not recommended and can cause unexpected behavior in queries and updates.

The issue occurs because MongoDB’s query parser expects dots to represent field path separators, not literal characters in field names.

Solution 1: Using Aggregation Operators

For MongoDB 4.4 and later, you can use the $getField and $setField aggregation operators to access fields with dots in their names:

const result = await collection.aggregate([
  {
    $match: {
      "mappedTaxonomies.orgId.attributes": { $exists: true }
    }
  },
  {
    $addFields: {
      fieldValue: {
        $getField: {
          field: "mappedTaxonomies.orgId.attributes.No. of items.value",
          input: "$mappedTaxonomies.orgId.attributes"
        }
      }
    }
  }
]).toArray();

Or using $literal to handle the field name with dots:

const result = await collection.aggregate([
  {
    $addFields: {
      fieldValue: {
        $getField: {
          field: { $literal: "No. of items.value" },
          input: "$mappedTaxonomies.orgId.attributes"
        }
      }
    }
  }
]).toArray();

The MongoDB documentation specifically mentions using $getField, $setField, and $literal to handle field names with periods and dollar signs.

Solution 2: Encoding Field Names

A practical approach is to encode dots in field names when storing them and decode them when accessing:

// When storing data, encode dots
function encodeFieldName(fieldName) {
  return fieldName.replace(/\./g, 'ENCODE_DOT');
}

// When accessing data, decode dots
function decodeFieldName(fieldName) {
  return fieldName.replace(/ENCODE_DOT/g, '.');
}

// Usage:
const encodedColumn = encodeFieldName(column); // 'NoENCODE_DOTofENCODE_DOTitems'
const key = `mappedTaxonomies.${orgId}.attributes.${encodedColumn}.value`;

// After retrieval, decode if needed
const decodedField = decodeFieldName(retrievedField);

This approach, mentioned in Stack Overflow discussions, replaces dots with a temporary placeholder that won’t interfere with MongoDB’s parsing.

Solution 3: Alternative Query Structure

You can restructure your data model to avoid the issue altogether:

// Instead of nested structure with dots, use a flat structure
const query = {
  "mappedTaxonomies.orgId.attributes": {
    $elemMatch: {
      "fieldName": "No. of items",
      "value": { $exists: true }
    }
  }
};

const result = await collection.find(query).toArray();

Or use a different separator in your field names:

// Replace dots with underscores or other characters
const column = column.replace(/\./g, '_'); // 'No_of_items'
const key = `mappedTaxonomies.${orgId}.attributes.${column}.value`;

Best Practices

1. Avoid dots in field names: The simplest solution is to avoid using dots in field names during database design.

2. Use encoding for legacy data: If you must work with existing data containing dots, use encoding/decoding.

3. Leverage aggregation operators: For MongoDB 4.4+, use $getField and related operators for reliable access.

4. Document your approach: Ensure your team understands the chosen method for handling dot-containing field names.

5. Consider data migration: For long-term solutions, consider migrating data to use alternative field names.

Complete Example

Here’s a complete example showing how to access your ‘No. of items’ field:

const MongoClient = require('mongodb').MongoClient;

async function getItemsField(orgId) {
  const client = await MongoClient.connect('mongodb://localhost:27017');
  const db = client.db('yourDatabase');
  const collection = db.collection('yourCollection');

  // Method 1: Using aggregation operators (MongoDB 4.4+)
  const result1 = await collection.aggregate([
    {
      $match: {
        "mappedTaxonomies": { $elemMatch: { orgId: orgId } }
      }
    },
    {
      $addFields: {
        fieldValue: {
          $getField: {
            field: { $literal: "No. of items.value" },
            input: { $arrayElemAt: ["$mappedTaxonomies.orgId.attributes", 0] }
          }
        }
      }
    }
  ]).toArray();

  // Method 2: Using encoded field names
  const encodedFieldName = "NoENCODE_DOTofENCODE_DOTitems";
  const result2 = await collection.find({
    [`mappedTaxonomies.${orgId}.attributes.${encodedFieldName}.value`]: { $exists: true }
  }).toArray();

  client.close();
  return { aggregationResult: result1, encodedResult: result2 };
}

// Usage
getItemsField('yourOrgId').then(console.log);

This approach gives you multiple options depending on your MongoDB version and data structure requirements. The aggregation method is the most robust for current MongoDB versions, while encoding provides a good workaround for existing data schemas.