$comment
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Definition
$commentThe
$commentquery operator associates a comment to any expression taking a query predicate.Because comments propagate to the
profilelog, adding a comment can make your profile data easier to interpret and trace.The
$commentoperator has the form:db.collection.find( { <query>, $comment: <comment> } )
Behavior
You can use the $comment with any expression taking a query
predicate, such as the query predicate in
db.collection.update() or in the $match stage of
the aggregation pipeline.
For an example, see Attach a Comment to an Aggregation Expression.
Examples
Attach a Comment to find
The following example adds a $comment to a
find() operation:
db.records.find( { 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" : [ 2, 0 ] }, "$comment" : "Find even values." }, "comment" : "Find even values.", ...
Comments also appear in the MongoDB log
if the database profiler level is set
to 2 and slowms is set to
0ms. This db.setProfilingLevel() command sets these two
parameters:
db.setProfilingLevel(2, 0)
The comment for the previous db.records.find() example
then appears as follows in the
MongoDB log:
{"t":{"$date":"2020-09-17T11:32:20.415-07:00"},"s":"I", "c":"COMMAND", "id":51803, "ctx":"conn7","msg":"Slow query", "attr":{"type":"command","ns":"test.records","appName":"MongoDB Shell","command":{"find":"records","filter":{"x":{"$mod":[2.0,0.0]}, "$comment":"Find even values."},"comment":"Find even values." ...
Attach a Comment to an Aggregation Expression
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" } } } ] )