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Description

This error occurs when a document field name starts with a dollar sign (`$`), which is reserved by MongoDB for system fields, operators, and variables. MongoDB strictly prohibits user-defined field names from beginning with `$`. It typically manifests during insert, update, or aggregation operations.

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Error Message

Dollar Prefixed Field Name

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Known Causes

3 known causes

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User-Defined Field Name Starts with '$'

Attempting to insert or update a document where a user-defined field name explicitly begins with the '$' character.

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Misinterpreting Update Operators

Incorrectly using a string starting with '$' as a literal field name within an update operation, rather than a valid MongoDB update operator.

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Reserved Field Name in Aggregation

Defining a new field within an aggregation pipeline stage (e.g., $project, $addFields) with a name that begins with '$'.

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Solutions

3 solutions available

1. Rename Fields in Documents easy

Identify and rename fields that start with a '$' character in your documents.

1

Connect to your MongoDB instance using the `mongosh` shell or a MongoDB GUI tool.

2

Identify documents containing fields with dollar prefixes. You can use `find()` with a regex for this. Replace `your_collection` with the actual collection name and `your_database` with your database name.

db.your_database.your_collection.find({ '$or': [ { '$regex': '^\$', '$options': 'i' } ] })

3

For each identified document, use the `updateMany()` method to rename the problematic field. This example renames a field named `$_oldField` to `_oldField`. You'll need to adapt the field names and potentially use a loop for multiple fields or documents.

db.your_database.your_collection.updateMany({ '_$oldField': { '$exists': true } }, { '$rename': { '_$oldField': '_oldField' } })

4

Alternatively, if you have a specific document ID, you can use `updateOne()`.

db.your_database.your_collection.updateOne({ '_id': ObjectId('your_document_id') }, { '$rename': { '_$oldField': '_oldField' } })

2. Modify Application Code medium

Update your application's data insertion and retrieval logic to avoid using dollar-prefixed field names.

1

Review your application's code that interacts with MongoDB. Look for any instances where you are creating or manipulating data structures that will be inserted into MongoDB, and ensure no field names begin with '$'.

2

In your data modeling, choose field names that are valid and do not start with a '$'. For example, instead of `$_id`, use `id` or `documentId` if you are not using MongoDB's default `_id` field.

3

If you are using an ORM or ODM, consult its documentation for best practices regarding field naming and how it maps to MongoDB.

4

After modifying your application code, redeploy it and re-insert or update the affected data to reflect the correct field naming conventions.

3. Data Migration with a Script advanced

Write a script to systematically rename problematic fields across your entire collection.

1

Create a JavaScript file (e.g., `rename_fields.js`) to house your migration logic.

const dbName = 'your_database';
const collectionName = 'your_collection';
const oldFieldName = '_$problematicField'; // The field name to rename
const newFieldName = 'problematicField'; // The desired new field name

async function renameFields() {
  const client = new MongoClient('mongodb://localhost:27017'); // Replace with your connection string
  try {
    await client.connect();
    const db = client.db(dbName);
    const collection = db.collection(collectionName);

    console.log(`Starting to rename '${oldFieldName}' to '${newFieldName}' in '${collectionName}' collection...`);

    const result = await collection.updateMany(
      { [oldFieldName]: { $exists: true } },
      { $rename: { [oldFieldName]: newFieldName } }
    );

    console.log(`Matched ${result.matchedCount} documents and modified ${result.modifiedCount} documents.`);
  } finally {
    await client.close();
  }
}

renameFields().catch(console.error);

2

Install the MongoDB Node.js driver if you haven't already.

npm install mongodb

3

Execute the script using Node.js. Ensure you have Node.js installed on your system.

node rename_fields.js

4

Verify the changes by querying your collection to confirm that the problematic fields have been renamed.

db.your_database.your_collection.findOne({ 'problematicField': { '$exists': true } })

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Related Errors

5 related errors

MongoDB Error 52: Illegal Dollar Field Names