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Array Update Operators

$push

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  • Definition
  • Compatibility
  • Syntax
  • Behavior
  • Modifiers
  • Examples

Definition

$push

The $push operator appends a specified value to an array.

Compatibility

You can use $push for deployments hosted in the following environments:

  • MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

  • MongoDB Enterprise: The subscription-based, self-managed version of MongoDB

  • MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB

Syntax

The $push operator has the form:

{ $push: { <field1>: <value1>, ... } }

To specify a <field> in an embedded document or in an array, use dot notation.

Behavior

In MongoDB 4.4 and earlier, update operators process document fields in lexicographic order. See Update Operators Behavior for details.

If the field is absent in the document to update, $push adds the array field with the value as its element.

If the field is not an array, the operation will fail.

If the value is an array, $push appends the whole array as a single element. To add each element of the value separately, use the $each modifier with $push. For an example, see Append a Value to Arrays in Multiple Documents. For a list of modifiers available for $push, see Modifiers.

Modifiers

You can use the $push operator with the following modifiers:

Modifier
Description
Appends multiple values to the array field.
Limits the number of array elements. Requires the use of the $each modifier.
Orders elements of the array. Requires the use of the $each modifier.
Specifies the location in the array at which to insert the new elements. Requires the use of the $each modifier. Without the $position modifier, the $push appends the elements to the end of the array.

When used with modifiers, the $push operator has the form:

{ $push: { <field1>: { <modifier1>: <value1>, ... }, ... } }

The processing of the $push operation with modifiers occur in the following order, regardless of the order in which the modifiers appear:

  1. Update array to add elements in the correct position.

  2. Apply sort, if specified.

  3. Slice the array, if specified.

  4. Store the array.

Examples

Append a Value to an Array

The following example appends 89 to the scores array:

db.students.update(
   { _id: 1 },
   { $push: { scores: 89 } }
)

Append a Value to Arrays in Multiple Documents

Add the following documents to the students collection:

db.students.insertMany( [
   { _id: 2, scores: [ 45, 78, 38, 80, 89 ] } ,
   { _id: 3, scores: [ 46, 78, 38, 80, 89 ] } ,
   { _id: 4, scores: [ 47, 78, 38, 80, 89 ] }
] )

The following $push operation appends 95 to the scores array in each document:

db.students.updateMany(
   { },
   { $push: { scores: 95 } }
)

To confirm that each scores array includes 95, run the following operation:

db.students.find()

The operation returns the following results:

[
   { _id: 1, scores: [ 44, 78, 38, 80, 89, 95 ] },
   { _id: 2, scores: [ 45, 78, 38, 80, 89, 95 ] },
   { _id: 3, scores: [ 46, 78, 38, 80, 89, 95 ] },
   { _id: 4, scores: [ 47, 78, 38, 80, 89, 95 ] }
]

Append Multiple Values to an Array

Use $push with the $each modifier to append multiple values to the array field.

The following example appends each element of [ 90, 92, 85 ] to the scores array for the document where the name field equals joe:

db.students.update(
   { name: "joe" },
   { $push: { scores: { $each: [ 90, 92, 85 ] } } }
)

Use $push Operator with Multiple Modifiers

A collection students has the following document:

{
   "_id" : 5,
   "quizzes" : [
      { "wk": 1, "score" : 10 },
      { "wk": 2, "score" : 8 },
      { "wk": 3, "score" : 5 },
      { "wk": 4, "score" : 6 }
   ]
}

The following $push operation uses:

  • the $each modifier to add multiple documents to the quizzes array,

  • the $sort modifier to sort all the elements of the modified quizzes array by the score field in descending order, and

  • the $slice modifier to keep only the first three sorted elements of the quizzes array.

db.students.update(
   { _id: 5 },
   {
     $push: {
       quizzes: {
          $each: [ { wk: 5, score: 8 }, { wk: 6, score: 7 }, { wk: 7, score: 6 } ],
          $sort: { score: -1 },
          $slice: 3
       }
     }
   }
)

The result of the operation is keep only the three highest scoring quizzes:

{
  "_id" : 5,
  "quizzes" : [
     { "wk" : 1, "score" : 10 },
     { "wk" : 2, "score" : 8 },
     { "wk" : 5, "score" : 8 }
  ]
}

Tip

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