I have a MongoDB collection with data in the format of:
[
{
"data1":1,
"data2":2,
"data3":3,
"data4":4,
"horses":[
{
"opponent":{
"jockey":"MyFirstName MyLastName",
"name":"MyHorseName",
"age":4,
"sex":"g",
"scratched":"false",
"id":"1"
},
"id":"1"
},
{
"opponent":{
"jockey":"YourFirstName YourLastName",
"name":"YourHorseName",
"age":4,
"sex":"m",
"scratched":"false",
"id":"2"
},
"id":"2"
}
]
},
...
]
Executing the following query returns exactly what I need:
db.race_results.find({ "$and": [ { "horses":
{ "$elemMatch": { "$and": [
{ "opponent.name": "MyFirstName MyLastName" },
{ "opponent.jockey": "MyHorseName"}
] } }
}
]})
However, this query takes 0.5 seconds to execute with my collection (there are a lot of records).
I am trying to find out how to create an index on the horses.opponent.name field of the data. I have read the docs about multikey indexes (here), but I'm not sure if this is exactly what I need or not. What I need (I think) is an index on the array element of horses, but only the name and jockey fields. Is this possible?
Is there a way to create an index to make my specific query (the one above) any faster?
Any pointers would be greatly appreciated. I am fairly new to MongoDB, but learning fast!
The index to create is:
db.race_results.ensureIndex({"horses.opponent.name":1, "horses.opponent.jockey":1})
After creating this index, the query in your case should return number of scanned objects that is equal to the number of matched objects:
db.race_results.find( { horses: { $elemMatch: { "opponent.name": "MyHorseName", "opponent.jockey": "MyFirstName MyLastName" } } }
).explain()
Related
Given collection:
{
"_id" : "1.1000038",
"recomendation" : [
"1.6739718"
]
}
/* 2 */
{
"_id" : "1.1000069",
"recomendation" : [
"1.9185509",
"1.9051998",
"1.9034279",
"1.8288046",
"1.8152670",
"1.858775",
"1.6224229",
"1.4591674",
"1.3862464",
"1.3427739",
"1.3080062",
"1.3003608",
"1.1694619",
"1.1634683",
"1.1590664",
"1.1524146",
"1.754599",
"1.700837",
"1.763617"
]
}
I need to query the MongoDB for a list of values and get the first element of the list of values
here is the query by mongo syntax
db.getCollection('similar_articles').find({"_id":{$in:["1.1000069","1.1000038"]}})
I don't want to filter it on the python side because it's can be too big.
I didn't find any documentation on it
desire output:
Pandas DataFrame
_id recom
1.1000038 1.6739718
1.1000069 1.9185509
I don't know pymongo so well, but you need this query:
First $match by _ids into the arreay (this is like the find you have).
And later use $project to create the field recom (you can use "recomendation" to overwrite the existing field) and set the value as the first into the array.
db.collection.aggregate([
{
"$match": { "_id": { "$in": [ "1.1000069", "1.1000038" ] } }
},
{
"$project": { "recom": { "$arrayElemAt": [ "$recomendation", 0 ] } }
}
])
Example here
Looking the doumentation it seems you only need to copy and paste this query.
given a document structure as shown, where the trades array can have thousands of items... how on earth could one do a query that would verify that the sequence always has 'startTradeId' one number higher than the previous items 'endTradeId', all the way through the array? is this even possible?
{
"name": "STOCK",
"trades": [{
"endTradeId": 41306,
"startTradeId": 41302,
...
},
{
"endTradeId": 41301,
"startTradeId": 41297,
...
},
{
"endTradeId": 41296,
"startTradeId": 41240,
...
},
...
]
}
You can use $where operator like below :
db.your_collection.find( { $where : function(){ return "this.trades.startTradeId > this.trades.endTradeId" }});
I have a data
[{ "name":"BS",
"keyword":"key1",
"city":"xyz"
},
{ "name":"AGS",
"keyword":"Key2",
"city":"xyz1"
},
{ "name":"QQQ",
"keyword":"key3",
"city":"xyz"
},
{ "name":"BS",
"keyword":"Keyword",
"city":"city"
}]
and i need to search records which have name= "BS" OR keyword="key2" with the help of query
db.collection.find({"$OR" : [{"name":"BS"}, {"keyword":"Key2"}]});
These records i need in the sequence
[{ "name":"BS",
"keyword":"key1",
"city":"xyz"
},
{ "name":"BS",
"keyword":"Keyword",
"city":"city"
},
{ "name":"AGS",
"keyword":"Key2",
"city":"xyz1"
}]
but i am getting in following sequences:
[{ "name":"BS",
"keyword":"key1",
"city":"xyz"
},
{ "name":"AGS",
"keyword":"Key2",
"city":"xyz1"
},
{ "name":"BS",
"keyword":"Keyword",
"city":"city"
}]
Please provide some suggestion i am stuck with this problem since 2 days.
Thanks
The order of results returned by MongoDB is not guaranteed unless you explicitly sort your data using the sort function. For smaller datasets you maybe "lucky" in the sense that the results are always returned in the same order, however, for bigger datasets and in particular when you have sharded Mongo clusters this is very unlikely. As proposed by Yathish you need to explicitly order your results using the sort function. Based on the suggested output, it seems you want to sort by name in descending order so I have set the sorting flag to -1 for the field name.
db.collection.find({"$or" : [{"name":"BS"}, {"keyword":"Key2"}]}).sort({"name" : -1});
If you need a more complex sorting algorithm as specified in your comment, you can convert your results to a Javascript array and create a custom sort function. This sort function will first list documents with a name equal to "BS" and then documents containing the keyword "Key2"
db.data.find({
"$or": [{
"name": "BS"
}, {
"keyword": "Key2"
}]
}).toArray().sort(function(doc1, doc2) {
if (doc1.name == "BS" && doc2.keyword == "Key2") {
return -1
} else if (doc2.name == "BS" && doc1.keyword == "Key2") {
return 1
} else {
return doc1.name < doc2.name
}
});
I'm sorry but I'm little confuse with a query , Kindly help. suppose we've one document that contains
{
"_id":100,
"name":"Demarcus Audette",
"scores":[
{
"score":47.42608580155614,
"type":"exam"
},
{
"score":44.83416623719906,
"type":"quiz"
},
{
"score":39.01726616178844,
"type":"homework"
},
"score":89.01726616178844,
"type":"homework"
}
]
}
And I want to write a query that should return only rows which contains homework in that , that means the out put should be like below
{
"_id":100,
"name":"Demarcus Audette",
"scores":[
{
"score":39.01726616178844,
"type":"homework"
},
"score":89.01726616178844,
"type":"homework"
}
]
}
Kindly suggest. Thanks in Advance
Use the $elemMatch operator.
db.collection.find({ "scores": { $elemMatch: { "type": "homework" } } } );
EDIT
What you are asking is not possible. You will need the above query and filter out the rest in whatever language you are programming. You can also use an aggregate function using $unwind and $match.
db.collection.aggregate(
{$unwind: "$messages"},
{$match: {"scores.type": "homework"}}
);
$unwind flattens your array and $match is your actually query which will return matching documents. Please note that $unwind will create a different document for each element in your array. This means you will get two results when you filter on 'homework' according to your example.
Mongo DB: I'm looking to make one query to return both the first and last element of an array. I realize that I can do this multiple queries, but I would really like to do it with one.
Assume a collection "test" where each objects has an array "arr" of numbers:
db.test.find({},{arr:{$slice: -1},arr:{$slice: 1}});
This will result in the following:
{ "_id" : ObjectId("xxx"), "arr" : [ 1 ] } <-- 1 is the first element
Is there a way to maybe alias the results? Similar to what the mysql AS keyword would allow in a query?
This is not possible at the moment but will be with the Aggregation Framework that's in development now if I understand your functional requirement correctly.
You have to wonder about your schema if you have this requirement in the first place though. Are you sure there isn't a more elegant way to get this to work by changing your schema accordingly?
This can be done with the aggregation framework using the operators $first and $last as follows:
db.test.aggregate([
{ '$addFields': {
'firstElem': { '$first': '$arr' },
'lastElem': { '$last': '$arr' }
} }
])
or using $slice as
db.test.aggregate([
{ '$addFields': {
'firstElem': { '$slice': [ '$arr', 1 ] },
'lastElem': { '$slice': [ '$arr', -1 ] }
} }
])