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The $comment query operator associates a comment to any expression taking a query predicate.

Because comments propagate to the profile log, adding a comment can make your profile data easier to interpret and trace.

The $comment operator has the form:

db.collection.find( { <query>, $comment: <comment> } )

You can use the $comment with any expression taking a query predicate, such as the query predicate in db.collection.updateOne() or in the $match stage of the aggregation pipeline. For an example, see Attach a Comment to an Aggregation Expression.

The following example adds a $comment to a find() operation:

x: { $mod: [ 2, 0 ] },
$comment: "Find even values."

If the Database Profiler is enabled, the following output shows the comment in the system.profile collection:

"op" : "query",
"ns" : "test.records",
"command" : {
"find" : "records",
"filter" : {
"x" : {
"$mod" : [
"$comment" : "Find even values."
"comment" : "Find even values.",

Comments also appear in the MongoDB log if the database profiler level is set to 2 and slowms is set to 0ms. This db.setProfilingLevel() command sets these two parameters:

db.setProfilingLevel(2, 0)

The comment for the previous db.records.find() example then appears as follows in the MongoDB log:

"c":"COMMAND", "id":51803, "ctx":"conn7","msg":"Slow query",
"$comment":"Find even values."},"comment":"Find even values."

You can use the $comment with any expression taking a query predicate.

The following example uses the $comment operator in the $match stage to clarify the operation:

db.records.aggregate( [
{ $match: { x: { $gt: 0 }, $comment: "Don't allow negative inputs." } },
{ $group : { _id: { $mod: [ "$x", 2 ] }, total: { $sum: "$x" } } }
] )
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