Working with Data - The Definitive Guide to MongoDB - page 58

The following example limits the items from the Cast list to the first three items:

> db.media.find({"Title" : "Matrix, The"}, {"Cast" : {$slice: 3}})
{ "_id" : ObjectId("4c5fcd3edb290000000067cb"), "Type" : "DVD", "Title" :
"Matrix, The", "Released" : 1999, "Cast" : [ "Keanu Reeves", "Carrie-Anne
Moss", "Laurence Fishburne" ] }

You can also get only the last three items by making the integer negative:

> db.media.find({"Title" : "Matrix, The"}, {"Cast" : {$slice: -3}})
{ "_id" : ObjectId("4c5fcd3edb290000000067cb"), "Type" : "DVD", "Title" :
"Matrix, The", "Released" : 1999, "Cast" : [ "Hugo Weaving", "Gloria Foster",
"Joe Pantoliano" ] }

Or you can skip the first two items and limit the results to three from that particular point (pay careful attention to
the brackets):

> db.media.find({"Title" : "Matrix, The"}, {"Cast" : {$slice: [2,3] }})
{ "_id" : ObjectId("4c5fcd3edb290000000067cb"), "Type" : "DVD", "Title" :
"Matrix, The", "Released" : 1999, "Cast" : [ "Laurence Fishburne", "Hugo
Weaving", "Gloria Foster" ] }

Finally, when specifying a negative integer, you can skip to the last five items and limit the results to four, as in
this example:

> db.media.find({"Title" : "Matrix, The"}, {"Cast" : {$slice: [-5,4] }})
{ "_id" : ObjectId("4c5fcd3edb290000000067cb"), "Type" : "DVD", "Title" :
"Matrix, The", "Released" : 1999, "Cast" : [ "Carrie-Anne Moss","Laurence
Fishburne","Hugo Weaving","Gloria Foster"] }

■ With version 2.4 MongoDB also introduced the $slice operator for $push operations, allowing you to limit the
number of array elements when appending values to an array. this operator is discussed later in this chapter. Do not
confuse the two, however.

Note

Searching for Odd/Even Integers
The $mod operator lets you search for specific data that consists of an even or uneven number. This works because the
operator takes the modulus of 2 and checks for a remainder of 0 , thereby providing even-numbered results only.
For example, the following code returns any item in the collection that has an even-numbered integer set to its
Released field:

> db.media.find ( { Released : { $mod: [2,0] } }, {"Cast" : 0 } )
{ "_id" : ObjectId("4c45b5c18e0f0000000062aa"), "Type" : "DVD", "Title" :
"Blade Runner", "Released" : 1982 }
{ "_id" : ObjectId("4c45b5df8e0f0000000062ab"), "Type" : "DVD", "Title" :
"Toy Story 3", "Released" : 2010 }