mongo $ slice查询反向索引超出范围 [英] mongo $slice query reverse index out of range
问题描述
以下在mongo中的查询的行为很奇怪:
The following query in mongo, behaves strange :
db.items.findOne({},{ "List": { "$slice": [ skip, 3 ] }})
首先: 它不返回仅使用["_id"，"List"]键的对象，而是返回完整的对象.
First: Instead of returning one object with ["_id","List"] keys only, it returns a full object.
第二:
如果skip为负数且|skip|高于list.length，则它返回前三个元素，就像skip==0
Second:
if skip is negative and |skip| is higher than list.length then it returns the first three elements as though skip==0
我期望:
{
"_id" : ObjectId("542babf265f5de9a0d5c2928"),
"List" : [
1,
2,
3,
4,
5
]
"other" : "not_important"
}
查询:
db.items.findOne({},{ "List": { "$slice": [-10, 3 ] }})
获得:
{
"_id" : ObjectId("542babf265f5de9a0d5c2928"),
"List" : []
}
相反，我得到:
{
"_id" : ObjectId("542babf265f5de9a0d5c2928"),
"List" : [
1,
2,
3
]
"other" : "not_important"
}
为什么?
我使用mongoDB 2.4.10
I use mongoDB 2.4.10
推荐答案
第二个:如果skip为负且| skip |高于list.length，则返回前三个元素，就好像skip == 0
是的.这就是mongodb内部使用的javascript Array.prototype.slice()方法的工作方式.
Yes. That is how the javascript Array.prototype.slice() method works, which is internally used by mongodb.
根据ECMAScript®语言规范 ，
如果relativeStart为负，则令k为max((len + relativeStart)，0); 否则让k为min(relativeStart，len).
If relativeStart is negative, let k be max((len + relativeStart),0); else let k be min(relativeStart, len).
对于您的情况relativeStart is -10，
k = max((-10+5),0), k = 0;(其中5是数组的长度).
In your case relativeStart is -10,
k = max((-10+5),0), k = 0; (where, 5 is the length of your array).
因此k或skip将始终为0.
首先:它不返回仅使用["_id"，"List"]键的对象，而是返回完整的对象.
是的，投影运算符以这种方式工作.除非在projection参数中明确指定了inclusion或exclusion，否则将使用诸如$slice，$elemmatch之类的投影运算符来检索整个文档.
Yes, the projection operator works that way. Unless a inclusion or exclusion is explicitly specified in the projection parameter, the whole document is retrieved with the projection operators such as $slice,$elemmatch being applied.
db.items.findOne({},{"_id":1,"List": { "$slice": [-10, 3 ] }})
将返回:
{ "_id" : ObjectId("542babf265f5de9a0d5c2928"), "List" : [ 1, 2, 3 ] }
findOne()方法的第二个参数是not only for simple projection用途，仅当field名称中的任何一个名称具有0或projection operator，则为applied和projected.
The second parameter to the findOne() method is not only for simple projection purpose, fields are not projected, only if any one of the field names have a value of 0 or 1 against them. If not the whole document is returned. If any field has a projection operator to be applied, it would be applied and projected.
只要涉及$slice运算符，投射机制似乎就会以以下方式发生.
The projection mechanism seems to happen in the below manner, whenever the $slice operator is involved.
- 默认情况下，所有字段都将包含在内以进行投影.
- 默认情况下，始终显示其值基于投影运算符
$slice，如果为真而导出的所有字段，而与以下内容无关.
- By default all the fields would be included for projection.
- By Default all the fields whose values are derived based on the projection operator,
$slice, if truthy, are always displayed, irrespective of the below.
- projection参数中指定的字段列表按其指定顺序累积.
- 仅遇到第一个值为'0'或'1'的字段: 如果 字段的值为'0'-然后将其排除，其余所有字段 字段被标记为包括在内. 如果字段具有"1"-则将其包括在内，其余所有字段 被标记为排除.
- 对于所有后续字段，根据以下内容排除或包含它们 他们的价值观.
- The list of fields specified in the projection parameter are accumulated in their specified order.
- For only the first field encountered with value '0' or '1': If the field has a value '0' - then it is excluded, and all the remaining fields are marked to be included. If a field has '1' - then it is included, and all the remaining fields are marked to be excluded.
- For all the subsequent fields, they are excluded or included based on their values.
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