Limiting Query result in MongoDB - mongodb

Question

I have 20,000+ documents in my mongodb. I just learnt that you cannot query them all in one go.
So my question is this:
I want to get my document using find(query) then limit its results for 3 documents only and I can choose where those documents start from.
For example if my find() query resulted in 8 documents :
[{doc1}, {doc2}, {doc3}, {doc4}, {doc5}, {doc6}, {doc7}, {doc 8}]
command limit(2, 3) will gives [doc3, doc4, doc5]
And I also need to get total count for all that result(without limit) for example : length() will give 8 (the number of total document resulted from find() function)
Any suggestion? Thanks

Answer 1

add .skip(2).limit(3) to the end of your query

Answer 2

I suppose you have the following documents in your collection.
{ "_id" : ObjectId("56801243fb940e32f3221bc2"), "a" : 0 }
{ "_id" : ObjectId("56801243fb940e32f3221bc3"), "a" : 1 }
{ "_id" : ObjectId("56801243fb940e32f3221bc4"), "a" : 2 }
{ "_id" : ObjectId("56801243fb940e32f3221bc5"), "a" : 3 }
{ "_id" : ObjectId("56801243fb940e32f3221bc6"), "a" : 4 }
{ "_id" : ObjectId("56801243fb940e32f3221bc7"), "a" : 5 }
{ "_id" : ObjectId("56801243fb940e32f3221bc8"), "a" : 6 }
{ "_id" : ObjectId("56801243fb940e32f3221bc9"), "a" : 7 }
From MongoDB 3.2 you can use the .aggregate() method and the $slice operator.
db.collection.aggregate([
{ "$group": {
"_id": null,
"count": { "$sum": 1 },
"docs": { "$push": "$$ROOT" }
}},
{ "$project": {
"count": 1,
"_id": 0,
"docs": { "$slice": [ "$docs", 2, 3 ] }
}}
])
Which returns:
{
"count" : 8,
"docs" : [
{
"_id" : ObjectId("56801243fb940e32f3221bc4"),
"a" : 2
},
{
"_id" : ObjectId("56801243fb940e32f3221bc5"),
"a" : 3
},
{
"_id" : ObjectId("56801243fb940e32f3221bc6"),
"a" : 4
}
]
}
You may want to sort your document before grouping using the $sort operator.
From MongoDB 3.0 backwards you will need to first $group your documents and use the $sum accumulator operator to return the "count" of documents; also in that same group stage you need to use the $push and the $$ROOT variable to return an array of all your documents. The next stage in the pipeline will then be the $unwind stage where you denormalize that array. From there use use the $skip and $limit operators respectively skip the first 2 documents and passes 3 documents to the next stage which is another $group stage.
db.collection.aggregate([
{ "$group": {
"_id": null,
"count": { "$sum": 1 },
"docs": { "$push": "$$ROOT" }
}},
{ "$unwind": "$docs" },
{ "$skip": 2 },
{ "$limit": 3 },
{ "$group": {
"_id": "$_id",
"count": { "$first": "$count" },
"docs": { "$push": "$docs" }
}}
])
As #JohnnyHK pointed out in this comment
$group is going to read all documents and build a 20k element array with them just to get three docs.
You should then run two queries using find()
db.collection.find().skip(2).limit(3)
and
db.collection.count()