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This article will discuss the $set and $setOnInsert operations. In addition, the issue of adding fields to a collection in MongoDB is also briefly explained using these two operators.

$set operator in MongoDB

The $set operator changes the value of a field to a supplied value. The $set operator expression is as follows: { $set: { <field1>: <value1>, ... } }.

Use dot notation to define inline text or arrays <field>.

Behavior of the $set Operator

Starting in MongoDB 5.0, update operators process document fields with string-based names in lexicographic order. Fields named with numbers are processed in numeric order.

If the field does not exist, $set will create one with the requested value, as long as the new field does not break type restrictions. If you provide a dotted path to a nonexistent field, $set will construct embedded documents as necessary to complete the dotted path.

If you provide multiple field-value pairs, $set will update or create each field.

mongod no longer throws an error when you use an update operator such as $set with an empty operand expression ( ) in MongoDB 5.0 {}. Since the update is empty, no modification is made and no oplog record is generated (meaning the operation is a no-op).

For example, let's create a collection of clothes:

db.clothes.insertOne(
   {
     _id: 100,
     quantity: 250,
     instock: true,
     reorder: false,
     details: { model: "14RR", make: "Clothes" },
     tags: [ "apparel", "clothing" ],
     ratings: [ { by: "Customer127", rating: 5 } ]
   }
)

Setting Top-Level Fields The following procedure uses the $set operator to change _idthe values ​​of the Quantity, Details, and Tags fields for documents that satisfy the condition equal to 100.

db.clothes.updateOne(
   { _id: 100 },
   { $set:
      {
        quantity: 400,
        details: { model: "2600", make: "Fashionables" },
        tags: [ "coats", "outerwear", "clothing" ]
      }
   }
)

This operation updates the following:

1. The quantity value is 400.
2. Details field for the new embedded document.
3. Label the fields with the new array.

The document now has the following values:

{
  _id: 100,
  quantity: 400,
  instock: true,
  reorder: false,
  details: { model: '2600', make: 'Fashionables' },
  tags: [ 'coats', 'outerwear', 'clothing' ],
  ratings: [ { by: 'Customer127', rating: 5 } ]
}

Setting fields in embedded documents

Use dot notation to represent embedded text or arrays <field>. The following operation modifies _idthe make field in the detail document of documents that satisfy the condition equal to 100:

db.clothes.updateOne(
   { _id: 100 },
   { $set: { "details.make": "Kidz" } }
)

After the update, the document now has the following values:

Set the elements in an array

Use dot notation to represent text or arrays embedded in the text <field>.

The following operation modifies the value in the second element of the tags field and the ratings field in the first element of the ratings array to satisfy the condition _idequal to 100 for the document.

db.clothes.updateOne(
   { _id: 100 },
   { $set:
      {
        "tags.1": "rain wear",
        "ratings.0.rating": 3
      }
   }
)

After the update, the document now has the following values:

$setOnInsert operator in MongoDB

If the document was inserted as a result of an update operation, upsert: true$ setOnInsert applies the provided value to the field in the document. If the update operation did not result in an insert, **$setOnInsert** does nothing.

The ups****ert option can be specified.

Behavior of the $setOnInsert operator

Starting in MongoDB 5.0, update operators process document fields with string-based names in lexicographic order. Fields named with numbers are processed in numeric order.

mongod no longer throws an error when you use an update operator such as $setOnInsert with an empty operand expression ( ) in MongoDB 5.0 {}. Since the update is empty, no modification is made and no oplog record is generated (meaning the operation is a no-op).

For example, the clothes collection contains no documents. We will insert a new document using insert with upsert: truethe parameter .db.collection.updateOne()

db.clothes.updateOne(
  { _id: 1 },
  {
     $set: { item: "banana" },
     $setOnInsert: { defaultQty: 100 }
  },
  { upsert: true }
)

<query>Used by MongoDB to create a _id: 1new document with the identifier $setOnInsert Makes the requested changes to the document.

Find the newly inserted document in the clothes collection:

{ "_id" : 1, "item" : "banana", "defaultQty" : 100 }

When the upsert parameter is true, db.collection.updateOne()the method:

1. Create a new document,
2. Apply the $set operation,
3. Apply the $setOnInsert action.

db.collection.updateOne()MongoDB only applies the $set operation if there is a match with an existing document.

Adding new fields to documents in a MongoDB collection

You can use the following method to add a new field to each document in a MongoDB collection.

Also shown are examples of using each method with a collection team with the following documents:

db.teams.insertOne({team: "United", position: "Defence", points: 31})
db.teams.insertOne({team: "Spurs", position: "Defence", points: 22})
db.teams.insertOne({team: "Palace", position: "Center", points: 19})
db.teams.insertOne({team: "Eagles", position: "Forward", points: 26})
db.teams.insertOne({team: "Lions", position: "Defence", points: 33})

Add a new field with no value

syntax:

db.collection.updateMany({}, {$set:{"new_field": null}})

You can use the following code to add a new field called rebounds with an empty value to every existing document in the collection:

db.teams.updateMany({}, {$set:{"rebounds": null}})

You can write the following query to view the first few updated documents:

db.teams.find().limit(3)

This query returns the following documents:

Add a new field with a specific value

syntax:

db.collection.updateMany({}, {$set:{"new_field": 10}})

You can use the following code to add a new field named rebounds with a value of 10 to every existing document in the collection:

db.teams.updateMany({}, {$set:{"rebounds": 10}})

You can write the following query to view the first few updated documents:

db.teams.find().limit(3)

This query returns the following documents:

Add a new field using the value from an existing field

syntax:

db.collection.updateMany(
    {},
    [
        {"$set": {"name": { "$concat": ["$field1", " ", "$field2"]}}}
    ]
)

You can use the following code to add a field called name whose value is the concatenation of the existing fields team and position.

db.teams.updateMany(
    {},
    [
        {"$set": {"name": { "$concat": ["$team", " ", "$position"]}}}
    ]
)

You can write the following query to view the first few updated documents:

db.teams.find().limit(3)

This query returns the following documents: