MongoDB Error – BadValue: can’t convert from BSON type string to Date

The Problem

If you’re working with MongoDB and you encounter the BadValue: can’t convert from BSON type string to Date error, it typically means that MongoDB has received a string in a place where it expects a date object. This usually happens during queries or when trying to insert or update documents with date fields. In this tutorial, we’ll explore common reasons for this error and provide several solutions to fix it.

Common Causes

The error occurs because MongoDB is strict about data types, especially for date fields. When a string that cannot be automatically converted to a date type is supplied, MongoDB throws this error. This might happen if the date string is not in an ISO format or if a miscellaneous string is mistakenly placed in a date field.

Solution 1: Convert string to Date in Application Code

The most straightforward approach is to convert string to Date object in your application code before sending it to MongoDB. This solution provides the most control over how the data is converted.

Steps:

• 1. Identify the fields that are causing the error.
• 2. Use your programming language’s date parsing capabilities to convert the string to a Date object.
• 3. Update your MongoDB operations to use the newly converted Date objects.

Code Example:

let dateString = "2023-04-01";
let dateObject = new Date(dateString);
console.log(dateObject); // Output: 2023-04-01T00:00:00.000Z

Notes: This method provides great flexibility but requires manual intervention. It’s crucial to ensure that the date string is in a format that can be correctly parsed.

Solution 2: Use MongoDB’s $dateFromString Aggregation Operator

MongoDB provides the $dateFromString aggregation operator, which can convert a string to a date within the query itself.

Steps:

• 1. Determine the aggregation pipeline where the conversion needs to happen.
• 2. Use the $dateFromString operator within your pipeline to convert the string to a Date object.
• 3. Execute the pipeline.

Code Example:

db.collection.aggregate([
  {
    $project: {
      convertedDate: {
        $dateFromString: {
          dateString: "$dateField"
        }
      }
    }
  }
]);

Notes: This solution is elegant for operations within MongoDB but might not be suitable if you need the conversion before reaching the database, such as in data validation scenarios.

Solution 3: Correct Data at Source

Sometimes, the best solution is to ensure that date strings are correctly formatted at their source.

Steps:

• 1. Identify the source of the data.
• 2. Ensure that dates are formatted correctly before they are sent to MongoDB.
• 3. Optionally, implement validation to prevent incorrectly formatted dates from being sent.

Notes: This solution prevents the issue from occurring but requires control over the data source, which might not always be possible.

Conclusion

The BadValue: can’t convert from BSON type string to Date error in MongoDB can be frustrating but is often easily remedied with the right approach. Whether through application-level data conversion, using MongoDB’s built-in operators, or ensuring accurate data at the source, resolving this issue helps maintain data integrity and functionality of your MongoDB operations.