Databases Reference
In-Depth Information
Unless otherwise specified, all keys in a document are returned when
"$slice"
is used.
This is unlike the other key specifiers, which suppress unmentioned keys from being
returned. For instance, if we had a blog post document that looked like this:
{
"_id" : ObjectId("4b2d75476cc613d5ee930164"),
"title" : "A blog post",
"content" : "...",
"comments" : [
{
"name" : "joe",
"email" : "joe@example.com",
"content" : "nice post."
},
{
"name" : "bob",
"email" : "bob@example.com",
"content" : "good post."
}
]
}
and we did a
"$slice"
to get the last comment, we'd get this:
> db.blog.posts.findOne(criteria, {"comments" : {"$slice" : -1}})
{
"_id" : ObjectId("4b2d75476cc613d5ee930164"),
"title" : "A blog post",
"content" : "...",
"comments" : [
{
"name" : "bob",
"email" : "bob@example.com",
"content" : "good post."
}
]
}
Both
"title"
and
"content"
are still returned, even though they weren't explicitly in-
cluded in the key specifier.
Querying on Embedded Documents
There are two ways of querying for an embedded document: querying for the whole
document or querying for its individual key/value pairs.
Querying for an entire embedded document works identically to a normal query. For
example, if we have a document that looks like this:
{
"name" : {
"first" : "Joe",
"last" : "Schmoe"
},
