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MongoDB positional operators $ or $[] do not work

MongoDB offers several update operators to modify values within array fields in documents. The positional operator ($) stands out for its ability to update specific elements in an array without explicitly specifying their position.

Today, we will examine the MongoDB positional operator ($) in detail, including its syntax, use cases, and best practices.

What is the MongoDB Positional Operator ($)?

The positional ($) operator is one of MongoDB’s array update operators, allowing updates to array fields, especially those containing embedded documents. This is handy when dealing with large datasets requiring selective modifications within arrays.

{ ".$": value }

This syntax indicates an update operation will be applied to an array’s first matching element based on a query condition.

Types of Positional Operators

MongoDB provides different types of positional operators to update array elements effectively:

1. $[] Operator

   The $[] operator, also known as the all positional operator, modifies all elements within the specified array field.

   { : { ".$[]" : value } }

   The $[] operator is useful when updating all elements in an array without iterating through them manually. It can be used in update operations such as `db.collection.updateOne()` and `db.collection.findAndModify()`.

   Here is an example:

   
db.collection.updateOne(
{ },
{ : { ".$[]" : value } }
)

2. $ Operator for Single Element Update

   The $ operator updates a single element in an array without explicitly specifying its position.

   { ".$" : value }

   The $ operator acts as a placeholder for the first matching element in the array. The array field must appear as part of the query document for it to work.

   Here is an example:

   
db.collection.updateOne(
{ "predictions.status": "FT" },
{ "$set": { "predictions.$.status": "Completed" } }
)


   This update modifies the first matching element in the `predictions` array where `status` is `FT`.

   Furthermore, MongoDB allows the $[] operator to work with nested arrays. This allows users to traverse and update values in deeply nested structures efficiently.

Solution to Common Issues

With the release of MongoDB 3.6, the $[] operator was introduced. If you run into issues with the positional operator, try these troubleshooting steps:

1. Make sure that the MongoDB version is 3.6 or later. If needed, we can set `FeatureCompatibilityVersion` to `3.6′.
2. The $ operator requires a filter for elements in the array. Here is an example query structure:

   const query = {'_id': new ObjectID(currentPrediction._id), "predictions.status": "FT"};

   Furthermore, the $ operator may not work as expected without a proper filter.
3. Remember to use array filters for specific element updates.

   Here is an example:

   
db.collection.updateOne(
{ "predictions.status": "FT" },
{ "$set": { "predictions.$[elem].status": "Completed" } },
{ arrayFilters: [ { "elem.status": "FT" } ] }
)


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Conclusion

The MongoDB positional operator ($) offers an easy way to update array elements efficiently. While the $[] operator modifies all elements, the $ operator targets a single element.

In brief, our Support Experts demonstrated how to proceed if MongoDB positional operators $ or $[] do not work.