【MongoDB学习笔记19】MongoDB的游标和查询选项
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 }
>
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 }
>
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；
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