Working with Data - The Definitive Guide to MongoDB - page 59

Likewise, you can find any documents containing an uneven value in the Released field by changing the
parameters in $mod , as follows:

> db.media.find ( { Released : { $mod: [2,1] } }, { "Cast" : 0 } )
{ "_id" : ObjectId("4c45b5b38e0f0000000062a9"), "Type" : "DVD", "Title" :
"Matrix, The", "Released" : 1999 }

■ the $mod operator only works on integer values, not on strings that contain a numbered value. For example,
you can't use the operator on { Released : "2010" } , because it's in quotes and therefore a string.

Note

Filtering Results with $size
The $size operator lets you filter your results to match an array with the specified number of elements in it. For
example, you might use this operator to do a search for those CDs that have exactly two songs on them:

> db.media.find ( { Tracklist : {$size : 2} } )
{ "_id" : ObjectId("4c1a86bb2955000000004076"), "Type" : "CD", "Artist" :
"Nirvana", "Title" : "Nevermind", "Tracklist" : [
{
"Track" : "1",
"Title" : "Smells Like Teen Spirit",
"Lenght" : "5:02"
},
{
"Track" : "2",
"Title" : "In Bloom",
"Length" : "4:15"
}
] }

■ You cannot use the $size operator to find a range of sizes. For example, you cannot use it to find arrays with
more than one element in them.

Note

Returning a Specific Field Object
The $exists operator allows you to return a specific object if a specified field is either missing or found. The following
example returns all items in the collection with a key named Author :

> db.media.find ( { Author : {$exists : true } } )

Similarly, if you invoke this operator with a value of false , then all documents that don't have a key named
Author will be returned:

> db.media.find ( { Author : {$exists : false } } )

■ Warning Currently, the $exists operator is unable to use an index; therefore, using it requires a full table scan.