MongoDB使用游标来处理find查询到的结果文档集；
游标的使用实例：
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> var cursor=db.post.
find
({
"name.firstname"
:
"joe"
})
> cursor
{
"_id"
: ObjectId(
"54ace1394ba07ed75df68f90"
),
"name"
: {
"firstname"
:
"joe"
,
"lastname"
:
"schome"
},
"age"
: 28 }
{
"_id"
: ObjectId(
"54ace753eab1d0ba4abb48ab"
),
"name"
: {
"firstname"
:
"joe"
,
"lastname"
:
"schome"
},
"age"
: 30 }
{
"_id"
: ObjectId(
"54ace777eab1d0ba4abb48ac"
),
"name"
: {
"firstname"
:
"joe"
,
"lastname"
:
"schome"
,
"comment"
: 10 },
"age"
: 30 }
{
"_id"
: ObjectId(
"54ace781eab1d0ba4abb48ad"
),
"name"
: {
"firstname"
:
"joe"
,
"lastname"
:
"schome"
,
"comment"
: 6 },
"age"
: 30 }
{
"_id"
: ObjectId(
"54ace785eab1d0ba4abb48ae"
),
"name"
: {
"firstname"
:
"joe"
,
"lastname"
:
"schome"
,
"comment"
: 5 },
"age"
: 30 }
{
"_id"
: ObjectId(
"54ace789eab1d0ba4abb48af"
),
"name"
: {
"firstname"
:
"joe"
,
"lastname"
:
"schome"
,
"comment"
: 7 },
"age"
: 30 }
>
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使用limit来返回前两个记录：
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> cursor.limit(2)
{
"_id"
: ObjectId(
"54ace1394ba07ed75df68f90"
),
"name"
: {
"firstname"
:
"joe"
,
"lastname"
:
"schome"
},
"age"
: 28 }
{
"_id"
: ObjectId(
"54ace753eab1d0ba4abb48ab"
),
"name"
: {
"firstname"
:
"joe"
,
"lastname"
:
"schome"
},
"age"
: 30 }
>
|
或者使用skip来跳过前3个记录：
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> cursor.skip(3)
{
"_id"
: ObjectId(
"54ace781eab1d0ba4abb48ad"
),
"name"
: {
"firstname"
:
"joe"
,
"lastname"
:
"schome"
,
"comment"
: 6 },
"age"
: 30 }
{
"_id"
: ObjectId(
"54ace785eab1d0ba4abb48ae"
),
"name"
: {
"firstname"
:
"joe"
,
"lastname"
:
"schome"
,
"comment"
: 5 },
"age"
: 30 }
{
"_id"
: ObjectId(
"54ace789eab1d0ba4abb48af"
),
"name"
: {
"firstname"
:
"joe"
,
"lastname"
:
"schome"
,
"comment"
: 7 },
"age"
: 30 }
>
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或者使用sort指定键值排序：
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> cursor.
sort
({
"age"
:1})
{
"_id"
: ObjectId(
"54ace1394ba07ed75df68f90"
),
"name"
: {
"firstname"
:
"joe"
,
"lastname"
:
"schome"
},
"age"
: 28 }
{
"_id"
: ObjectId(
"54ace753eab1d0ba4abb48ab"
),
"name"
: {
"firstname"
:
"joe"
,
"lastname"
:
"schome"
},
"age"
: 30 }
{
"_id"
: ObjectId(
"54ace777eab1d0ba4abb48ac"
),
"name"
: {
"firstname"
:
"joe"
,
"lastname"
:
"schome"
,
"comment"
: 10 },
"age"
: 30 }
{
"_id"
: ObjectId(
"54ace781eab1d0ba4abb48ad"
),
"name"
: {
"firstname"
:
"joe"
,
"lastname"
:
"schome"
,
"comment"
: 6 },
"age"
: 30 }
{
"_id"
: ObjectId(
"54ace785eab1d0ba4abb48ae"
),
"name"
: {
"firstname"
:
"joe"
,
"lastname"
:
"schome"
,
"comment"
: 5 },
"age"
: 30 }
{
"_id"
: ObjectId(
"54ace789eab1d0ba4abb48af"
),
"name"
: {
"firstname"
:
"joe"
,
"lastname"
:
"schome"
,
"comment"
: 7 },
"age"
: 30 }
|
综合使用sort/limit/skip,当然可以调整顺序得到不同的结果：
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> cursor.
sort
({
"name.comment"
:1}).skip(2).limit(1)
{
"_id"
: ObjectId(
"54ace785eab1d0ba4abb48ae"
),
"name"
: {
"firstname"
:
"joe"
,
"lastname"
:
"schome"
,
"comment"
: 5 },
"age"
: 30 }
>
|
游标生命周期：
游标会消耗内存和其他资源，游标使用完后会释放资源以供数据库它用；游标会在以下情况终止，
（1）游标完成匹配结果的迭代后，它会清楚自身；
（2）客户端的游标已经不再作用域内，驱动程序回向服务器发送一条特别的消息，让其销毁；
（3）游标在十分钟内没有使用，游标自动销毁；
本文转自 bannerpei 51CTO博客，原文链接：http://blog.51cto.com/281816327/1600283，如需转载请自行联系原作者
