MongoDB tip: Using the positional $ operator in array

This post is a quick documentation on how to update elements in arrays using positional operator to look for the value to update.

Let's setting up our example:

db.movies.insertMany( [
  {

    "title" : "Batman",
    "category" : [ "action", "adventure" ],
    "imdb_rating" : 7.6,
    "budget" : 35

},
{



     
    "title" : "Godzilla",
    "category" : [ "action",
    "adventure", "sci-fi" ],
    "imdb_rating" : 6.6

},
{



     
    "title" : "Home Alone",
    "category" : [ "family", "comedy" ],
    "imdb_rating" : 7.4

}
])

From MongoDB documentation:

The positional $ operator

• $ is a positional operator that specifies an element in an array to update.
• It acts as a placeholder for the first element that matches the query document. 
• $ replaces the element in the specified position with the value given. 
• Example:
  db.<collection>.updateOne(
  {<array> : value ... },
  {<update operator> : { "<array>.$" : value } } )
  

Using .$ here

// the "action" category needs to be changed to "action-adventure"

     
db.movies.updateMany( { "category": "action",  },
                      { $set: { "category.$" : "action-adventure" } } )

Now we can see every document which contains the category with value "action" is replaced with "action-adventure"