How to return an error when $addToSet does not add an item because it already exists in Mongoose?

In Mongoose, the $addToSet operator is used to add an element to an array if it doesn't already exist. If the element already exists in the array, the $addToSet operation will not add a duplicate and will not raise an error by default. Instead, it will silently ignore the operation.

If you want to be notified when the $addToSet operation does not add an item due to duplication, you can follow these steps:

1. Use the $addToSet operator as usual to update the array field in your Mongoose model.

2. After the update operation, check the nModified property in the update result. If nModified is equal to 0, it means that the update did not modify any documents, indicating that the item already exists in the array.

Here's a code example demonstrating this approach:

Assuming you have a Mongoose model called MyModel with an array field named myArray, and you want to add a new item to the array:

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const MyModel = require('./path/to/your/model'); // Replace with the actual path to your Mongoose model

async function addToMyArray(itemToAdd) {
  try {
    const updateResult = await MyModel.updateOne(
      { _id: 'your_document_id' }, // Replace with the ID of the document you want to update
      { $addToSet: { myArray: itemToAdd } }
    );

    // Check if the update modified any documents (nModified === 1) or not (nModified === 0)
    if (updateResult.nModified === 1) {
      console.log('Item added successfully!');
    } else {
      console.log('Item already exists in the array.');
      // You can throw an error here or handle it as needed
      // throw new Error('Item already exists in the array.');
    }
  } catch (error) {
    console.error('Error occurred:', error.message);
    // Handle the error as needed
  }
}

// Call the function with the item you want to add to the array
addToMyArray('new item');

In this example, addToMyArray is a function that uses Mongoose's updateOne method with the $addToSet operator to add the new item to the array in the specified document. After the update, it checks the nModified property in the updateResult to determine if the update added the item or not. If nModified is 1, it means the item was added successfully. If nModified is 0, it means the item already exists in the array, and you can choose to handle it accordingly, such as throwing an error or logging a message.