mongodb $slice array value repeat use skip limit not take effect?

[陈龙]

for example
document {'id':1,'array':[1,2,3,4,4,4,4]}
use db.test.find({'id':1},{'array':{'$slice':[-1,3]}}) ,but result array
[4],why?
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[Stephen Steneker]

Post by 陈龙
for example
document {'id':1,'array':[1,2,3,4,4,4,4]}
use db.test.find({'id':1},{'array':{'$slice':[-1,3]}}) ,but result array
[4],why?

Hi,

The negative slice is returning items with the skip counting from the end
of the array:
http://docs.mongodb.org/manual/reference/operator/projection/slice/

Your example is confusing since you have multiple values of "4" in your
array.

What you've asked for is effectively "skip to the item that is 1 position
from the end of the array, and return the next 3 array items including that
one".

To make it clearer try using unique values in your test document:
db.test.insert({id:1, array:[1,2,3,4,5,6,7]})


Now you will see that your slice with a skip of -1 and limit of 3 returns
the last element (as expected):
db.test.find({'id':1},{'array':{'$slice':[-1,3]}})
{
"_id" : ObjectId("535879ae930332c0db35e886"),
"id" : 1,
"array" : [
7
]
}

The limit of 3 isn't useful here as there are no elements following the
last one in the array.

For comparison, try taking a slice skipping to the 3rd-to-last element in
the array and limiting to the following 2 items:
db.test.find({'id':1},{'array':{'$slice':[-3,2]}})
{
"_id" : ObjectId("535879ae930332c0db35e886"),
"id" : 1,
"array" : [
5,
6
]
}

Regards,
Stephen
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[陈龙]

Thank you!
array elements not repeat is ok, but i test array repeat state, I did not
expect,why array element repeat is that

圚 2014幎4月24日星期四UTC+8䞊午10时53分14秒Stephen Steneker写道

Post by Stephen Steneker

Post by 陈龙
for example
document {'id':1,'array':[1,2,3,4,4,4,4]}
use db.test.find({'id':1},{'array':{'$slice':[-1,3]}}) ,but result array
[4],why?

Hi,
The negative slice is returning items with the skip counting from the end
http://docs.mongodb.org/manual/reference/operator/projection/slice/
Your example is confusing since you have multiple values of "4" in your
array.
What you've asked for is effectively "skip to the item that is 1 position
from the end of the array, and return the next 3 array items including that
one".
db.test.insert({id:1, array:[1,2,3,4,5,6,7]})
Now you will see that your slice with a skip of -1 and limit of 3 returns
db.test.find({'id':1},{'array':{'$slice':[-1,3]}})
{
"_id" : ObjectId("535879ae930332c0db35e886"),
"id" : 1,
"array" : [
7
]
}
The limit of 3 isn't useful here as there are no elements following the
last one in the array.
For comparison, try taking a slice skipping to the 3rd-to-last element in
db.test.find({'id':1},{'array':{'$slice':[-3,2]}})
{
"_id" : ObjectId("535879ae930332c0db35e886"),
"id" : 1,
"array" : [
5,
6
]
}
Regards,
Stephen

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