$max
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Definition
$maxThe
$maxoperator updates the value of the field to a specified value if the specified value is greater than the current value of the field. The$maxoperator can compare values of different types, using the BSON comparison order.The
$maxoperator expression has the form:{ $max: { <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 does not exists, the $max operator sets the
field to the specified value.
Examples
Use $max to Compare Numbers
Consider the following document in the collection scores:
{ _id: 1, highScore: 800, lowScore: 200 }
The highScore for the document currently has the value
800. The following operation uses $max to compare
the 800 and the specified value 950 and updates the value
of highScore to 950 since 950 is greater than 800:
db.scores.update( { _id: 1 }, { $max: { highScore: 950 } } )
The scores collection now contains the following modified document:
{ _id: 1, highScore: 950, lowScore: 200 }
The next operation has no effect since the current value of the
field highScore, i.e. 950, is greater than 870:
db.scores.update( { _id: 1 }, { $max: { highScore: 870 } } )
The document remains unchanged in the scores collection:
{ _id: 1, highScore: 950, lowScore: 200 }
Use $max to Compare Dates
Consider the following document in the collection tags:
{ _id: 1, desc: "crafts", dateEntered: ISODate("2013-10-01T05:00:00Z"), dateExpired: ISODate("2013-10-01T16:38:16.163Z") }
The following operation compares the current value of the
dateExpired field, i.e.
ISODate("2013-10-01T16:38:16.163Z"), with the specified date
new Date("2013-09-30") to determine whether to update the
field:
db.tags.update( { _id: 1 }, { $max: { dateExpired: new Date("2013-09-30") } } )
The operation does not update the dateExpired field:
{ _id: 1, desc: "decorative arts", dateEntered: ISODate("2013-10-01T05:00:00Z"), dateExpired: ISODate("2013-10-01T16:38:16.163Z") }