So far we have learned how to Create, Read, and Update data using different methods provided by MongoDB. Let’s explore the last CRUD operation, Delete.

There are certain situations when data is no longer necessary or becomes obsolete. MongoDB provides us a couple options to permanently remove unwanted documents from a collection. In this exercise, we’ll focus on learning how to use the .deleteOne() method to remove a single document from a collection.

In order to use .deleteOne(), we must provide specific filtering criteria to find the document we want to delete. MongoDB will look for the first document in the collection that matches the criteria and delete it. Let’s take a closer look at the syntax:

db.<collection>.deleteOne(<filter>, <options>);

Note in the syntax above, the .deleteOne() method takes two arguments:

• filter: A document that provides selection criteria for the document to delete.
• options: A document where we can include optional fields to provide more specifications to our operation, such as a writeConcern.

To see .deleteOne() in action, consider a collection, monsters, with the following documents:

{
  _id: ObjectId(...),
  name: "Luca",
  age: 100,
  type: "Hydra"
},
{
  _id: ObjectId(...),
  name: "Lola",
  age: 95,
  type: "Hydra"
},
{
  _id: ObjectId(...),
  name: "Igor",
  age: 95,
  type: "Chimera"
},

If we want to delete a single monster with an age of 95, we can run the following command:

db.monsters.deleteOne({ age: 95 });

If the command is successful, MongoDB will confirm the document was deleted with the following output:

{ acknowledged: true, deletedCount: 1 }

The collection would then be left with these remaining documents:

{
  _id: ObjectId(...),
  Name: "Luca",
  age: 100,
  type: "Hydra"
},
{
  _id: ObjectId(...),
  name: "Igor",
  age: 95,
  type: "Chimera"
},

Notice that only one of the documents in the collection with the age of 95 was deleted. When the filter criteria is non-unique, the document that gets deleted is the first one that MongoDB identifies when performing the operation. Which document is found first depends on several factors which can include insertion order and the presence of indexes relevant to the filter.

In the following exercises, we’ll practice using the .deleteOne() method with the same collection from the previous lesson, restaurants.